Self-suspending proppants for hydraulic fracturing comprising a coating of hydrogel-forming polymer

ABSTRACT

The invention provides for modified proppants, comprising a proppant particle and a hydrogel coating, wherein the hydrogel coating localizes on the surface of the proppant particle to produce the modified proppant, methods of manufacturing such proppants and methods of use.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/599,828 filed Aug. 30, 2012, which claims the benefit of U.S.Provisional Application Ser. No. 61/529,600, filed Aug. 31, 2011, U.S.Provisional Application Ser. No. 61/635,612 filed Apr. 19, 2012, andU.S. Provisional Application Ser. No. 61/662,681, filed Jun. 21, 2012;this application also claims the benefit of U.S. Provisional ApplicationNo. 61/635,612, filed Apr. 19, 2012, U.S. Provisional Application Ser.No. 61/662,681, filed Jun. 21, 2012, U.S. application Ser. No.13/599,828 filed Aug. 30, 2012, U.S. Provisional Application Ser. No.61/725,751, filed Nov. 13, 2012 and U.S. Provisional Application Ser.No. 61/764,792 filed Feb. 14, 2013. The entire teachings of the aboveapplications are incorporated herein by reference.

FIELD OF APPLICATION

This application relates generally to systems, formulations and methodsfor fracturing technologies.

BACKGROUND

In the process of acquiring oil and/or gas from a well, it is oftennecessary to stimulate the flow of hydrocarbons via hydraulicfracturing. The term “fracturing” refers to the method of pumping afluid into a well until the pressure increases to a level that issufficient to fracture the subterranean geological formations containingthe entrapped materials. This process results in cracks and breaks thatdisrupt the underlying layer to allow the hydrocarbon product to becarried to the well bore at a significantly higher rate. Unless thepressure is maintained, however, the newly formed openings close. Inorder to open a path and maintain it, a propping agent or “proppant” isinjected along with the hydraulic fluid to create the support needed topreserve the opening. As the fissure is formed, the proppants aredelivered in a slurry where, upon release of the hydraulic pressure, theproppants form a pack or a prop that serves to hold open the fractures.

To accomplish the placement of the proppants inside the fracture, theseparticles are suspended in a fluid that is then pumped to itssubterranean destination. To prevent the particles from settling, a highviscosity fluid is often required to suspend them. The viscosity of thefluid is typically managed by addition of synthetic or naturally-basedpolymers. There are three common types of polymer-enhanced fluid systemsin general use for suspending and transporting proppants duringhydraulic fracturing operations: slickwater, linear gel, and crosslinkedgel.

In slickwater systems, an anionic or cationic polyacrylamide istypically added as a friction reducer additive, allowing maximum fluidflow with a minimum of pumping energy. Since the pumping energyrequirements of hydraulic fracturing are high, on the order of10,000-100,000 horsepower, a friction reducer is added to slickwaterfluids to enable high pumping rates while avoiding the need for evenhigher pumping energy. While these polymers are effective as frictionreducers, they are not highly effective as viscosifiers and suspendingagents. Slickwater polymer solutions typically contain 0.5-2.0 gallonsof friction reducer polymer per 1000 gallons of slickwater fluid, andthe solutions have low viscosity, typically on the order of 3-15 cps. Atthis low viscosity, suspended proppant particles can readily settle outof suspension as soon as turbulent flow is stopped. For this reason,slickwater fluids are used in the fracturing stages that have either noproppant, proppant with small particle size, or low proppant loadings.

The second type of polymer enhanced fluid system is known as a lineargel system. Linear gel systems typically contain carbohydrate polymerssuch as guar, hydroxyethylcellulose, hydroxyethyl guar, hydroxypropylguar, and hydroxypropylcellulose. These linear gel polymers are commonlyadded at a use rate of 10-50 pounds of polymer per 1000 gallons oflinear gel fluid. These concentrations of linear gel polymer result in afluid with improved proppant suspending characteristics vs. theslickwater fluid. The linear gel fluids are used to transport proppants,at loading levels of about 0.1 to 1 pound of proppant per gallon offluid. Above this proppant loading level, a more viscous solution istypically required to make a stable suspension.

Crosslinked gel is the most viscous type of polymer-enhanced fluid usedfor transporting of proppant. In crosslinked gel systems, the linear gelfluid as described above is crosslinked with added reagents such asborate, zirconate, and titanate in the presence of alkali. Uponcrosslinking of the linear gel fluid into a crosslinked gel fluid, theviscosity is much higher and the proppants can be effectively suspended.The linear gel and crosslinked gel fluids have certain advantages butthey require a high dose rate of expensive polymer.

Modifications of proppant particles could be used advantageously toimprove their performance in hydraulic fracturing systems. First, if theproppant particles were more buoyant, a less viscous suspension fluidcould be used, which would still convey the particles to the target areabut which would be easier to pump into the formation. Second, it isdesirable that the proppants remain where they are placed throughout thelifetime of the well after they have been injected into a fracture line.If changes within the reservoir during well production force theproppants out of position, production equipment can be damaged, and theconductivity of the reservoir formation can be decreased as thereservoir pores are plugged by the displaced proppants. Third, theproppants in the system should be resistant to closure stress once theyare placed in the fracture. Closure stresses can range from 1700 psi incertain shale gas wells, up to and exceeding 15,000 psi for deep, hightemperature wells. Care must be taken that the proppants do not failunder this stress, lest they be crushed into fine particles that canmigrate to undesirable locations within the well, thereby affectingproduction. Desirably, a proppant should resist diagenesis duringfracture treatment. The high pressures and temperatures combine with thechemicals used in frac fluids can adversely affect the proppantparticles, resulting in their diagenesis, which can eventually producefine particulate matter that can scale out and decrease the productivityof the well over time.

Current proppant systems and polymer-enhanced fracturing fluids endeavorto address these concerns, so that the proppants can be carried by thefracturing fluids, can remain in place once they arrive at their targetdestination, and can resist the closure stresses in the formation. Oneapproach to preparing suitable proppants includes coating the proppantmaterials with resins. A resin-coated proppant can be either fully curedor partially cured. The fully cured resin can provide crush resistanceto the proppant substrate by helping to distribute stresses among thegrain particles. A fully cured resin can furthermore help reduce finemigration by encapsulating the proppant particle. If initially partiallycured, the resin may become fully cured once it is placed inside thefracture. This approach can yield the same benefits as the use of aresin that is fully-cured initially. Resins, though, can decrease theconductivity and permeability of the fracture, even as the proppants areholding it open. Also, resins can fail, so that their advantages arelost. Resin-based systems tend to be expensive and they are still proneto settling out of suspension.

In addition, there are health, safety and environmental concernsassociated with the handling and processing of proppants. For example,fine particulates (“fines”), such as crystalline silica dust, arecommonly found in naturally occurring sand deposits. These fines can bereleased as a respirable dust during the handling and processing ofproppant sand. With chronic exposure, this dust can be harmful toworkers, resulting in various inhalation-associated conditions such assilicosis, chronic obstructive pulmonary disease, lung cancers in thelike. In addition to these health effects, the fines can cause “nuisancedust” problems such as fouling of equipment and contamination of theenvironment.

Another approach to preparing suitable proppants involves mixingadditives with the proppant itself, such as fibers, elastomericparticles, and the like. The additives, though, can affect therheological properties of the transport slurry, making it more difficultto deliver the proppants to the desired locations within the fracture.In addition, the use of additives can interfere with uniform placementof the proppant mixture into the fracture site. While there are knownmethods in the art for addressing the limitations of proppant systems,certain problems remain. There is thus a need in the art for improvedproppant systems that allow precise placement, preserve fractureconductivity after placement, protect well production efficiency andequipment life, simplify hydraulic fracturing operations, reduceenvironmental impact, and promote worker health and safety. It isfurther desirable that such improved systems be cost-effective.

SUMMARY OF THE INVENTION

The invention relates to modified proppants, comprising a proppantparticle and a hydrogel coating, wherein the hydrogel coating localizeson the surface of the proppant particle to produce the modifiedproppant. The proppant particles can be solids such as sand, bauxite,sintered bauxite, ceramic, or low density proppant. Alternatively oradditionally, the proppant particle comprises a resin-coated substrate.Optionally, the modified proppant further comprises an adhesionpromoter, optionally affixing the hydrogel coating to the resin-coatedsubstrate. The hydrogel coating preferably comprises a water-swellablepolymer. The hydrogel coating can be manufactured form a water solublepolymer. The preferred weight average molecular weight of the polymer is≧about 1 million g/mol, preferably ≧about 5 million g/mol. Preferably,the proppant is dry, free-flowing when dry and/or free-flowing afterbeing subjected to a relative humidity of between about 80%-90% for onehour at 25-35° C. The hydrogel coating is preferably durable andpossesses a shearing ratio as determined by a Shear Analytical Test of≧0.6.

The invention relates to methods of manufacturing the proppants and tothe proppants produced by the methods. Preferably, the hydrogel coatingis applied to the proppant particle as a liquid coating formulation thatdries to form a substantially continuous film on the surface of theproppant particle. The modified proppant can be made by an invertemulsion coating technique in which the proppant particle substrate iscombined with an invert emulsion in which the oil phase forms thecontinuous phase of the emulsion and a solution or dispersion of thesuperabsorbent polymer in water forms the discontinuous, emulsifiedphase.

The hydrogel coating preferably comprises a polymer selected from thegroup consisting of polyacrylamide, hydrolyzed polyacrylamide,copolymers of acrylamide with ethylenically unsaturated ioniccomonomers, copolymers of acrylamide and acrylic acid salts,poly(acrylic acid) or salts thereof, carboxymethyl cellulose,hydroxyethyl cellulose, hydroxypropyl cellulose, guar gum, carboxymethylguar, carboxymethyl hydroxypropyl guar gum, hydrophobically associatingswellable emulsion polymers, and latex polymers. The amount of hydrogelcoating can be less than about 5 wt % of the total dry weight.

The modified proppant is preferably self-suspending. Preferred proppantsof the invention can undergo a volumetric expansion of at least 100%,preferably at least 500%, upon hydration in an excess of water.

The modified proppants can comprise additional excipients, such as acationic/anionic polymer pair comprising a cationic polymer and a highmolecular weight anionic polymer. The cationic polymer can be selectedfrom the group consisting of poly-DADMAC, LPEI, BPEI, chitosan, andcationic polyacrylamide.

The modified proppants are preferably used in conjunction with and/orfurther comprise an oxidative breaker or an enzymatic breaker. Theoxidative breaker can be selected from the group consisting ofperoxides, magnesium peroxide, calcium peroxide, persulfate salts,nitrate salts, bromate salts, ozone, and oxidizing chlorine species. Theoxidative breaker can be a cationically modified oxidative breakercapable of associating with the hydrogel by ionic interaction. Theenzymatic breaker can be a cationic enzymatic breaker capable ofassociating with the hydrogel by ionic interaction. The modifiedproppant can further comprise a hydrophobic outer layer. For example,the hydrophobic outer layer can be selected from the group consisting offatty acids, aliphatic amines, hydrophobic quaternary amines, aliphaticamides, hydrogenated oils, vegetable oils, castor oil, triacetin, waxes,polyethylene oxides, and polypropylene oxides. Alternatively, themodified proppant can further comprise a delayed hydration additive,such as a low hydrophilic-lipophilic balance surfactant, an exclusionagent capable of excluding a finishing surfactant, an ionic crosslinkingagent, a covalent crosslinking agent and/or a monovalent salt chargeshielder. The modified proppant can further comprise an alcohol selectedfrom the group consisting of ethylene glycol, propylene glycol,glycerol, propanol, and ethanol. In an embodiment, the modified proppantof claim 1, further comprises an anticaking agent, such as a hydrophobiclayer material, a finely divided particulate material and/or acrosslinking agent. Examples of anticaking agent include calciumsilicate, calcium carbonate, talc, kaolin, bentonite, diatomaceousearth, silica, colloidal silica, microcrystalline cellulose, andattapulgate. They can also include fumed silica, calcium silicate,calcium carbonate, kaolin, bentonite and attapulgate. The hydrogelcoating can comprise an additive, such as a chemical additive or tracer.

The modified proppant preferably contains less fines than a proppantparticle that is not modified.

The invention includes hydraulic fracturing formulations, comprising themodified proppants described herein and an oxidative breaker or anenzymatic breaker. The invention also includes methods of fracturingwells. Such methods preferably comprise the steps:

preparing the hydraulic fracturing formulation of the invention, and

introducing the hydraulic fracturing formulation into the well in aneffective volume and at an effective pressure for hydraulic fracturing,

thereby fracturing the well.

In embodiments, the method of fracturing a well, comprises:

preparing a hydraulic fracturing formulation comprising the modifiedproppant of the invention,

introducing the hydraulic fracturing formulation into the well in aneffective volume and at an effective pressure for hydraulic fracturing,

providing a breaker formulation comprising an oxidative breaker or anenzymatic breaker, and

adding the breaker formulation into the well at an effective volume andat an effective volume,

thereby fracturing the well.

In the methods the breaker formulation can be added into the wellbefore, during or after introducing the hydraulic fracturing formulationinto the well. The breaker formulation can be added in one or moresteps.

In a process for fracturing a geological formation penetrated by a wellin which a fracing fluid containing a proppant is charged into thegeological formation with pulsed pressure, the invention includes amethod for reducing the amount of thickening agent that is added to thefracing fluid comprising selecting as the proppant the modified proppantof the invention. The modified proppants of the invention preferablyhydrate essentially completely within 2 hours, such as within 10minutes, of first being combined with the fracing fluid.

The invention includes methods of manufacturing a modified proppant.Such methods can comprise the steps:

providing a proppant substrate particle and a fluid polymeric coatingcomposition; and

applying the fluid polymeric coating composition on the proppantsubstrate particle;

optionally drying the modified proppant;

wherein the fluid polymeric coating composition comprises a hydrogelpolymer, and wherein the hydrogel polymer localizes on the surface ofthe proppant substrate particle to produce the modified proppant. Thestep of drying can dry the fluid polymeric coating so as to form asubstantially continuous film on the surface of the modified proppant.The method can preferably take place at or near a point of use for themodified proppant, such as a location which produces sand, ceramic, lowdensity proppant, a resin coated substrate, and/or bauxite. The methodcan further comprise adding an alcohol selected from the groupconsisting of ethylene glycol, propylene glycol, glycerol, propanol, andethanol during or before the step of mixing the proppant substrateparticles and the fluid polymer coating composition.

The method preferably comprises adding an inversion promoter during orfollowing the step of mixing the proppant substrate particles and thefluid polymer coating composition and/or an anticaking agent.

The methods of manufacturing a hydrogel-coated proppant can alsocomprise:

providing a proppant substrate particle and a formulation comprising acoating precursor, wherein the coating precursor is capable of forming ahydrogel coating on a surface of the proppant substrate particle by insitu polymerization;

applying the formulation to the proppant substrate particle; and

polymerizing the coating precursor in juxtaposition to the proppantsubstrate particle to form the hydrogel-coated proppant.

The method preferably results in a substantially continuous coating filmon the surface of the proppant substrate particle.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows vials of uncoated sand (L) and hydrogel coated sand (middleand R) in water.

FIGS. 2A-2C shows microscope images of the time dependent hydration ofthe hydrogel layer on a proppant.

FIG. 3 is a flow diagram of a manufacturing process for self-suspendingproppants.

FIG. 4 (FIGS. 4A and 4B) shows SEM images of proppant particles coatedwith hydrogel, without addition of glycerol (FIG. 4A) and with theaddition of glycerol (FIG. 4B).

FIG. 5 shows a SEM image of dried hydrogel coating on the surface ofproppant particle.

FIG. 6 is a graph of bed height vs. shear time for three sets ofself-suspending proppant samples.

FIG. 7 is a graph of bed height vs. mixing time for two sets ofself-suspending proppant samples.

FIG. 8 is a graph of bed height vs. mixing time for two sets ofself-suspending proppant samples.

FIG. 9 is a graph of bed height vs. mixing time for a series of treatedself-suspending proppant samples.

FIG. 10 is a graph of bed height for varying amounts of calcium silicateadded to self-suspending proppant samples.

FIG. 11 is a graph of bed height vs. drying time for a series ofpreheated and non-preheated proppant samples.

FIG. 12 shows a graph of bed height vs. drying time at varioustemperatures.

FIG. 13 shows a graph of temperature vs. mixing time for a series oftreated self-suspended proppant samples.

FIG. 14 shows a graph of bed height and loss of ignition (LOI) vs.drying time.

DETAILED DESCRIPTION 1. Modified Proppant Particles

Disclosed herein are systems and methods for forming and using proppantparticles having a hydrogel surface layer to enhance the hydrodynamicvolume of the proppant particles during fluid transport, creating a morestable proppant suspension that resists sedimentation, separation, andscreenout before the proppant can reach the intended target destinationin the fracture. Further benefits of the hydrogel-coated proppants asdisclosed herein include lower tendency to erode equipment, lowerfriction coefficient in the wet state, good bonding adhesion with eachother after placement in a fracture site, resistance to uncontrolledfines formation, and anti-fouling properties attributable to thehydrophilic surface. In embodiments, the disclosed systems for formingproppant particles can be applied to the types of proppant substratesmost widely used, e.g., sand, resin coated sand, bauxites, low densityproppants, and ceramics. In other embodiments, the proppant particlescan be formed from a variety of substrates, including fibrous materials,as would be available to those having ordinary skill in the art. Incertain embodiments, the proppant particles can be fabricated so thatthey resist crush or deformation, so that they resist displacement, andso that they can be suspended in less viscous fluid carriers fortransporting into the formation.

The invention encompasses a modified proppant, comprising a proppantparticle and a hydrogel coating, wherein the hydrogel coating localizeson the surface of the proppant particle to produce the modifiedproppant. In embodiments, these self-suspending proppants are formed bymodification of a particulate substrate with a water swellable polymercoating such as a hydrogel. In embodiments, the particulate substratecan be modified with the polymer coating before the particulatesubstrate is introduced into the fracturing fluid. In embodiments, theamount of hydrogel polymer coating can be in the range of about 0.1 toabout 10% based on the weight of the proppant. In embodiments, thehydrogel layer applied onto the surface of the proppant substrate can bea coating thickness of about 0.01% to about 20% of the average diameterof the proppant substrate. Upon hydration and swelling of the hydrogellayer in the fracturing fluid, the hydrogel layer can become expandedwith water, such that the expanded hydrogel layer thickness can becomeabout 10% to about 1000% of the average diameter of the proppantsubstrate. FIG. 1 shows an image of three vials, each containing thesame amount of proppant in water, where the vial on the left containsproppant with no hydrogel coating, the vial in the center containsproppant with 1% hydrogel coating, and the vial on the right containsproppant with 3% hydrogel coating. In each vial, the proppant was mixedwith water and then allowed to settle for 24 hours without agitation.The settled bed volume of the hydrogel coated proppants is significantlylarger than the settled bed volume of the uncoated proppant, showingthat the hydrogel coated proppant remains suspended in water. FIGS. 2A,2B and 2C show, respectively, three light microscopy images of the samehydrogel-coated proppant grain, where each image was taken after adifferent amount of time hydrating the hydrogel-coated proppant inwater. In FIG. 2A, the hydrogel-coated proppant particle had been inwater for 15 seconds; in FIG. 2B, the hydrogel-coated proppant particlehad been in water for 45 seconds; in FIG. 2C, the hydrogel-coatedproppant particle had been in water for 120 seconds. As shown in theseFigures, the hydrogel layer grows in volume quickly and expandssignificantly in size as the hydration time increases.

While it is known in the art to form hydrogel coatings on individualproppant particle substrates by coating them with superabsorbentpolymers (see, for example, U.S. 2008/0108524), the formulations andmethods disclosed herein differ from such technologies in important,advantageous ways. As disclosed herein, hydrogel formulations that areused have certain salient properties. In more detail, the formulationsdisclosed herein comprise hydrogels that are selected and applied to aproppant particle, forming a modified particle that, in a way so that:(a) when dry, is free flowing, and/or, (b) upon hydration with water,the hydrogel coating is durable and/or the hydrogel coating expandsvolumetrically so that the volume of the hydrated modified proppant isat least 20% greater than the volume of the dry modified proppant, or isbetween about 20% to about 50% greater than the volume of the drymodified proppant, or is between about 50% and about 100% greater thanthe volume of the dry modified proppant, or is between about 100% andabout 200% greater than the volume of the dry modified proppant, or isbetween about 200% and about 400% greater than the volume of the drymodified proppant, or is greater than about 400% of the volume of thedry modified proppant.

As the term “dry” is used herein, a modified proppant will be understoodto be dry when its moisture content is 1 wt. % or less. Preferably, themoisture content of the modified proppants of this disclosure, when dry,is ≦0.5 wt. %, or even ≦0.1 wt. %. In embodiments, the thickness of thedried hydrogel coating on the modified proppant can be less than 10microns, and often less than 2 microns. In embodiments, the hydration ofthe hydrogel polymers in an aqueous suspension is essentially completewithin 2 hours, or within 1 hour, or within 30 minutes, or within 10minutes, or within 2 minutes or even within 1 minute of being contactedwith an excess of tap water at 20° C. As used herein, describing ahydrogel coated proppant as being “hydrated essentially completely”means that the amount of volume increase by the hydrogel coated proppantis at least 80% of the total amount of volume increase by the hydrogelcoated proppant when hydrated completely in water.

In embodiments, modified proppants formed in accordance with theseformulations and methods, will, when dry, be free flowing, with anyclumping or agglomeration being readily dispersed by gentle agitation.The modified proppants shall still be considered to be free-flowing ifthey exhibit some degree of clumping or agglomeration, provided thatthese clumps and agglomerates can be broken up by gentle agitation.

The volumetric expansion of the proppants can be determined using aSettled Bed Height Analytical Test. For example, in a 20 mL glass vial,1 gm of the dry modified proppant to be tested is added to 10 gms ofwater (e.g., tap water) at approximately 20° C. The vial is thenagitated for about 1 minute (e.g., by inverting the vial repeatedly) towet the modified proppant coating. The vial is then allowed to sit,undisturbed, until the hydrogel polymer coating has become hydrated. Theheight of the bed formed by the hydrated modified proppant can bemeasured using a digital caliper. This bed height is then divided by theheight of the bed formed by the dry proppant. The number obtainedindicates the factor (multiple) of the volumetric expansion. Also, forconvenience, the height of the bed formed by the hydrated modifiedproppant can be compared with the height of a bed formed by uncoatedproppant, as shown in the following working Example 5.

Coating durability can be measured following the Shear Analytical Test.For example, 1 L of water (e.g., tap water) is added to a square 1 Lbeaker (such a beaker having a total volume of approximately 1.25 L,with the fill line at the 1 L mark). The beaker is then placed in an ECEngineering CLM4 paddle mixer. The mixer is set to mix at 300 rpm. Oncemixing commences, 50 gm of the modified proppant to be tested, in dryform, is added to the beaker. After 30 seconds of mixing at 300 rpm, themixing rate is reduced to 200 rpm and mixing is continued until thehydrogel polymer coating is hydrated. The mixture is then poured into agraduated 1 L cylinder and allowed to settle, after which the settledbed height of the modified proppant is measured in the manner indicatedabove. This settled bed height (“settled bed height with shearing”) isthen compared with the settled bed height of an identical amount ofhydrated modified proppant which has not been subjected to this sheartreatment (“settled bed height without shearing”). The amount by whichthis shear treatment reduces the settled bed height of the modifiedproppant is a measure of the durability of its hydrogel coating. For thepurposes of this disclosure, a hydrogel coating is considered durable ifthe ratio of the settled bed height with shearing to the settled bedheight without shearing (“shearing ratio”) is at least 0.2. Modifiedproppants exhibiting shearing ratios of greater than 0.2, greater thanor equal to 0.3, greater than or equal to 0.4, greater than or equal to0.5, greater than or equal to 0.6, greater than or equal to 0.7, greaterthan or equal to 0.8, or greater than or equal to 0.9 are desirable.

As indicated above, the type and amount of hydrogel polymer used in themodified proppants disclosed herein can be selected so that thevolumetric expansion of the modified proppant, as determined by theabove Settled Bed Height Analytical Test, increases by a factor of atleast 1.2. In particular embodiments, as shown in working Example 5,this factor can be greater than or equal to about 3, about 5, about 7,about 8 and even about 10.

As also indicated above, the modified proppants of this disclosure arefree flowing when dry. In particular embodiments, these modifiedproppants are free flowing even after being subjected to high humidityconditions, such as would be found, for example, in a mid-summer's dayin the southern United States. For this purpose, a modified proppant tobe tested can be subjected to humidity test conditions of 80-90%relative humidity at 25-50° C. for 1 hour. A modified proppant that isstill free flowing after being subjected to these humidity testconditions is regarded as being free flowing even after being subjectedto high humidity conditions.

Methods for modification of proppant include spraying or saturation of aliquid polymer formulation onto a proppant substrate, followed by dryingto remove water or other carrier fluids. The drying process can beaccelerated by application of heat or vacuum, and by tumbling oragitation of the modified proppant during the drying process. Theheating can be applied by forced hot air, convection, friction,conduction, combustion, exothermic reaction, microwave heating, orinfrared radiation. Agitation during the proppant modification processhas a further advantage of providing a more uniform coating on theproppant material.

FIG. 3 illustrates schematically a manufacturing process 100 forpreparing self-suspending proppant 130 in accordance with the presentdisclosure. In the depicted embodiment, sand 132 (e.g., dry sand havingless than 0.1% moisture) is conveyed via a conveyor belt 122 into amixing vessel 124, and a liquid polymer composition 120 is sprayed viapump and spray nozzle apparatus 134 onto the sand 132 along the conveyorbelt 122. The sand 132 exposed to the liquid polymer 120 reports to alow shear mixing vessel 124, where the ingredients are further blendedto form modified sand 128. After mixing, the modified sand containingthe liquid polymer is sent to a dryer 126 to remove water and/or organiccarrier fluids associated with the liquid polymer 120. After the dryingstep, the dried modified sand 132 is passed through a finalizing step134, which can include a shaker and/or other size classificationequipment such as a sieve to remove over-sized agglomerates. Thefinalizing step 134 can also subject the dried modified sand 132 tomechanical mixers, shear devices, grinders, crushers or the like, tobreak up aggregates to allow the material to pass through theappropriate sized sieve. The finished material 130 is then stored forshipment or use.

In embodiments, the sand or other substrate that is used to produceself-suspending proppant is pre-dried to a moisture content of <1%, andpreferably <0.1% before being modified with a hydrogel polymer. Inembodiments, the sand or other substrate temperature at the time ofmixing with the liquid polymer is in the range of about 10 to about 200degrees C., and preferably in the range of about 15 to about 80 degreesC. or between 15 and 60 degrees C.

In embodiments, the proppant substrate is contacted with the liquidpolymer composition by means of spraying or injecting. The amount ofliquid polymer composition added is in the range of about 1 to about20%, and preferably about 2 to about 10% by weight of the sand. Theproppant substrate and liquid polymer are blended for a period of 0.1 to10 minutes. In a preferred embodiment, the mixing equipment is arelatively low shear type of mixer, such as a tumbler, vertical conescrew blender, v-cone blender, double cone blender, pug mill, paddlemixer, or ribbon blender. In embodiments, the mixing equipment can beequipped with forced air, forced hot air, vacuum, external heating, orother means to cause evaporation of the carrier fluids.

In embodiments, the modified proppant substrate containing the liquidpolymer is dried to remove water and/or organic carrier fluidsassociated with the liquid polymer. The dryer equipment can be aconveyor oven, microwave, or rotary kiln type. In an embodiment thedrying step is carried out in such a way that the dried, modified sandcontains less than 1% by weight of residual liquids, including water andany organic carrier fluids associated with the liquid polymercomposition.

In embodiments, the same equipment can be used to blend the proppantsubstrate with the liquid polymer and to dry the blended product in asingle processing stage, or in a continuous production line.

In other embodiments, methods for modification of proppant includesynthesis of a hydrogel coating in situ, or in the presence of theproppant particle, resulting in a hydrogel layer encapsulating thesurface of the proppant particle. As an example, the in situ synthesisof the hydrogel can be accomplished by combining proppant particles withcoating precursor monomers and/or macromonomers followed by apolymerization step. In other exemplary instances, a water-solublepolymer can be dissolved in monomers, with or without solvent, followedby polymerization in the presence of the proppant particles, resultingin the formation of interpenetrating polymer networks as a coating onthe proppants. In other exemplary instances, the water-soluble polymeris dispersed in the monomers, with or without solvent, and thesubsequent polymerization will result in proppants encapsulated by ahydrogel consisting of water-soluble polymer particles locked up by thenewly formed polymer. The monomers or macromonomers used can be selectedfrom monomers that result in water-soluble polymers. In other exemplaryinstances, the particles can be encapsulated by non-water solublepolymer that will then be modified or hydrolyzed to yield thewater-soluble hydrogel coating. As would be understood by those ofordinary skill in the art, the encapsulating layer can be formed bydifferent polymerization techniques, with or without solvents. The insitu polymerization of polymer on the surface of proppant grains canhave the advantage of reducing or eliminating drying steps.

By way of example, a water-soluble monomer(s) for the hydrogel coatingor the in situ polymerization can be chosen from the following monomersor salts thereof: acrylic acid, methacrylic acid, acrylamide,methacrylamide, and their derivatives, carboxyethyl acrylate,hydroxyethylmethacrylate (HEMA), hydroxyethylacrylate (HEA),polyethyleneglycol acrylates (PEG-acrylates), N-isopropylacrylamide(NiPA), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), sodium saltof styrene sulfonate, vinylsulphonic acid, (meth)allylsulphonic acid,vinylphosphonic acid, N-vinylacetamide, N-methyl-N-vinylacetamide,N-vinylformamide, N-methyl-N-vinylformamide, N-vinylpyrrolidone,N-butyrolactam or N-vinylcaprolactam, maleic anhydride, itaconic acid,vinyl acetate, dimethyldiallylammonium chloride; quaternizeddimethylaminoethyl methacrylate (DMAEMA),(meth)acrylamidopropyltrimethylammonium chloride, methylvinylimidazoliumchloride; 2-vinylpyridine; 4-vinylpyridine, and the like. The ratio ofionic to nonionic monomers can be selected to yield hydrogels withdifferent charge density. In some instances, for example, it isdesirable to have hydrogels with higher charge in order to yieldcoatings with faster hydration or swelling properties. In an embodiment,the ionic content or charge density of the hydrogel polymer is in therange of 10-70% ionic, with the balance nonionic, on a molar percentbasis of the monomers. In a preferred embodiment, the charge density ofthe hydrogel polymer is in the range of 25-55% on a molar percent basis.In other instances the ionizable monomers can be selected to have higheror lower ionization constants to yield hydrogels more or less stable inbrine environments. Other advantageous properties can be imparted byselection of appropriate charge densities.

In embodiments, coating precursors can include polyfunctional monomersthat contain more than one polymerizable group and that will introducethe crosslinking or branching points in the hydrogel. Examples of thesemonomers are: pentaerythritol triallyl ether, PEG-diacrylates andmetahcrylates, N,N′-methylenebisacrylamide, epichlorohydrin, divinylsulfone, and glycidyl methacrylate. When such monomers are used, thecrosslinking monomer will be in the range 0.001 to 0.5% of the totalmonomer content. In selecting a range for adding crosslinkers, oneshould be aware that adding excessive amounts of crosslinker, forexample amounts greater than the 0.001 to 0.05% of total monomer contentamount, could form brittle hydrogels that can fracture or degrade underpressure. In embodiments, adding crosslinkers can form hydrogels lesslikely to become detached from the surface particle under extremeconditions.

In embodiments the monomers/macromonomers used are selected from coatingprecursor monomers that that will form a non-water soluble coating.After the coating is applied, its further modification will result inthe water swellable polymer. As an example, a polymeric coatingcontaining hydrolysable groups can be formed, and subsequent hydrolysiswill yield the hydrogel. Examples of monomers that fall in this categoryare esters, anhydrides, nitriles, and amides; for example the estermonomers methyl acrylate, t-butyl acrylate can be used. As anotherexample, a monomer containing vinyl functionalities can form thehydrogel by different polymerization techniques with or withoutsolvents. The polymerization techniques include bulk, suspension,admicellar, solution polymerization.

In other embodiments, coating monomers or precursors can be selected toform a self-suspending proppant with a hydrogel comprising apolyurethane or polyurea. A list of suitable monomers to form polymerswith polyurethane and/or polyurea functionalities are: polyols such asethylene glycol, propylene glycol, glycerin, trimethylolpropane,1,2,6-hexanetriol, pentaerythritol, sorbitol, sucrose,a-methylglycoside, polyoxyalkylenes such as PEG, copolymers of PEG-PPG,Pluronics, Tetronics, polyamines such as Jeffamines. Among theisocyanates there may be mentioned toluene-diisocyanate,naphthalenediisocyanate, xylene-diisocyanate, tetramethylenediisocyanate, hexamethylene diisocyanate, trimethylene diisocyanate,trimethyl hexamethylene diisocyanate, cyclohexyl-1,2-diisocyanate,cyclohexylene-1,4-diisocyanate and the like. Other appropriate polymerscan include HYPOL® hydrophilic polyurethane prepolymers from Dow,DESMODUR® and MONDUR® resins from Bayer(2,4′-diphenylmethanediisocyanate, 4,4′-diphenylmethanediisocyanate, andtheir mixtures), and CONATHANE® (polyisocyanate functionalizedprepolymers of toluene diisocyanate and poly(tetramethylene glycols))from Cytec, and the like.

The coating of proppant particle with a polyurethane (PU) hydrogel canbe carried out by conventional methods. In an embodiment, the coatingcan be performed in bulk without the use of solvents. For example, atypical formulation for a crosslinked PU hydrogel can be prepared in aone-step bulk polymerization process using a diisocyanate,polyoxyalkylene, and a multifunctional crosslinking agent. In anembodiment, the formulation will contain 10 to 80% of a polyoxyalkylenehaving the polyoxyalkylene molecular weight between 200 and 25,000.

Another method to form the hydrogel layer in situ can be carried out bydissolving or suspending a water-soluble polymer in a monomerformulation followed by polymerization of the monomer. The monomers canbe selected form the previous list of water soluble monomers. In thecase that the water-soluble polymer is dissolved in the monomer mixture,the resulting coating will consist in interpenetrating hydrogel networkof the initial water-soluble polymer and the polymer formed in situ. Inthe case where the water-soluble polymer is suspended in the monomermixture, the resulting coating will consist of a hydrogel coating inwhich the water soluble particles are locked up or entrapped. Forexample, these particles can be trapped inside the newly formed hydrogelcoating or they can be bonded to the newly formed polymer. Thewater-soluble polymer can be dissolved or suspended in the monomerformulation in the presence or absence of a solvent and thepolymerization can be carried out by different techniques.

Suitable water soluble polymers to be mixed with monomers can beselected from the group consisting of polyacrylamide, polyacrylic acid,copolymers of acrylamide with acrylic acid salts, polyethyleneglycol,polyvinylpyrrolidone, polyvinylalcohol, carboxymethyl cellulose,hydroxyethyl cellulose, hydroxypropyl cellulose, guar gum, carboxymethylguar, carboxymethyl hydroxypropyl guar gum, hydrophobically associatingswellable emulsion polymers, starches, latex polymers, and the like.

Another method for modification of proppant particles includeschemically grafting hydrophilic polymers onto the particle. The graftingof polymer chains onto the surface of the particle can be done byreactions such as Huisgen cycloaddition and other coupling or additionreactions that can immobilize the polymers onto the particle surface.

The proppant particle used for these purposes can be selected to havesurface functional groups such as epoxy, vinyl, amine, hydroxyl, etc.Those groups can then react with polymers having groups capable ofreacting with the functional groups on the particle surface. Forexample, proppant particles comprising silica can be surface modified bysilanes such as aminosilanes, vinylsilanes, epoxysilanes, etc.

In embodiments, the polymers that will react with the functionalizedparticle are hydrophilic linear or branched polymers or copolymers. Thepolymer can have one or more grafting moiety. In embodiments, thepolymers can have functional groups such as amino, carboxyl or saltsthereof, hydroxyl, thiol, acid anhydride, acid chloride and/orisocyanate groups which enable covalent binding to the functional groupsof the particle. Examples of polymers that can be used to react with thefunctionalized particle are: epoxide functionalized PEG, aminefunctionalized PEG, azide functionalized PEG, polyethyleneimine,polyacrylic acid, polyvinyl alcohol, etc.

In embodiments the resulting hydrogel, in addition to having swellableproperties, can have temperature responsive or pH-responsive properties.The resulting swellable properties of the proppant can thus be tuned.This is an added benefit for transporting proppant down the wellbore,since temperatures are lower at the early stages in which proppant istransported and full swelling behavior is desirable; higher temperaturesare expected inside the fractures where lower swelling of the hydrogellayer is desirable for packing improvement. The monomers used to makethe temperature responsive hydrogel coated proppants can be selectedfrom N-isopropylacrylamide (NiPA), ethylene oxide, propylene oxide, ormacromonomers/polymers that display a lower critical solutiontemperature (LCST).

In an embodiment, the process of converting a substrate such as sandinto a self-suspending proppant can be conducted at or near the point ofuse, for example at an oil or gas well site in preparation for hydraulicfracturing. This method has the advantage of converting a commoditymaterial with high material handling costs, such as sand, into aspecialized material that has added features. The sand can be acquiredfrom local sources or shipped directly from a sand mining site orwarehouse, for modification at the point of use. This avoids having toship sand first into a blending plant and then ship a second time fromthe blending plant to the point of use. In the case of sand, theshipping costs can be higher than the material costs, so avoidance ofextra shipping is desirable for controlling costs.

In an exemplary manufacturing process, the sand and the modifyingchemicals can be added to a continuous mixer. After mixing is complete,the mixture can either be (a) ready to use, or (b) sent to a dryingstep. The drying step can include a thermal or vacuum drying process,and it can include the addition of anticaking agents. The finishedproduct can be stored in containers at the well site. An example of themixing equipment is a continuous ribbon blender or a pug mill. Thedrying step can be a separate process from mixing, and the drying stepcan be designed to avoid overshearing of the finished product such as aconveyor or tunnel dryer. Other types of drying mechanisms includerotary kilns, microwave driers, paddle driers, and vacuum driers.

Hydrogel polymers that can be used to modify proppants in accordancewith the systems and methods disclosed herein can be introduced, inembodiments, as oil-based emulsions, suspensions, water-based emulsions,latexes, solutions, and dispersions. In embodiments, the hydrogelpolymers can be introduced as a distilled emulsion, such as an oil basedemulsion that has been partially evaporated to remove a portion of thecarrier fluids. This can offer the advantage of reduced dryingrequirements compared with conventional emulsions. In embodiments, thehydrogel polymer can be an alkali-swellable emulsion, wherein thehydrogel properties of the polymer are not fully developed until thepolymer is contacted with alkali. In this embodiment, thealkali-swellable emulsion can be coated onto the proppant substrate toform a modified proppant, and this modified proppant can be suspended ina fracturing fluid in the presence of an alkaline material.

In embodiments, an additive such as an alcohol selected from the groupconsisting of ethylene glycol, propylene glycol, glycerol, propanol, andethanol can be added during or before the step of mixing the proppantsubstrate particles and the liquid polymer coating composition. Inembodiments, inversion promoters useful as additives in the polymercoating formulations for self-suspending proppants can include high HLBsurfactants, such as polyethylene oxide lauryl alcohol surfactant,(ETHAL LA-12/80% from ETHOX), ethylene glycol, propylene glycol, water,sodium carbonate, sodium bicarbonate, ammonium chloride, urea, bariumchloride, and mixtures thereof. In embodiments, inversion promoters canserve the function of facilitating the release of active polymeringredients from the internal phase of an oil based emulsion polymerinto the (typically aqueous) process fluid to be treated. Since thisprocess converts an oil continuous polymer into a water continuousenvironment, can be characterized as a phase inversion.

In other embodiments, the proppant substrate can be modified with apolymer formulation, without the need for a drying step. This can beaccomplished by the use of a solvent-free polymer formulation, or acurable formulation. In certain simplified methods, a dry or liquidpolymer formulation can be applied onto the proppant substrate viainline mixing, and the modified material thus prepared can be usedwithout further processing. The moisture content of the proppantsubstrate can be modified by addition or removal of water, or additionof other liquids, to allow the substrate to be effectively coated,handled, and delivered into the fracturing fluid.

The modified proppants can be further modified with a wetting agent suchas a surfactant or other hydrophilic material to allow for effectivedispersion into a fracturing fluid. When the hydrogel-modified proppantsare suspended into a fracturing fluid, they are considered to beself-suspending if they require a lower viscosity fluid to prevent theparticles from settling out of suspension.

The modified proppants can be further modified to improve flowabilityand handling properties during processing, transport and storage. Thehygroscopic surface of the modified proppants can in some casesnegatively impact the bulk solids flow of the modified proppants bycausing the modified proppants to agglomerate, especially evident inmoist and/or high humidity environments. Anticaking properties can beimparted to the modified proppants through further modification toreduce or eliminate agglomeration by reducing the hygroscopic tendencyof the modified proppants during processing, transport, and storage orby reducing the interaction of surfaces between adjacent modifiedproppants during processing, transport and storage, or both. Inembodiments, the anticaking agent does not impact the intendedperformance of the modified proppants once the modified proppants areadded to an aqueous fluid in end use applications. The modifiedproppants can be treated with anticaking agents such as finely dividedsolids or a second outer layer or both. The second outer layer can be alow level of crosslinking of the modified proppant surface, or a solidnon-hygroscopic layer, or a cationic salt layer, or an oily hydrophobiclayer or a combination thereof. The modified proppants with theanticaking agent can have improved handling properties, such asfree-flowing properties, resistance to clumping, ease of conveying, easeof metering, and ease of discharging from a storage or transport vessel.In embodiments, the modified proppants with the anticaking agents canhave reduced drying requirements, so that the finished product can beproduced with a reduced amount of energy, time, and equipment.

In embodiments, the anticaking agent is a finely divided solidcomprising clays, siliceous materials, organics, metal oxides or fattyacid salts. In other embodiments, the anticaking agent is a finelydivided solid such as calcium silicate, magnesium silicate, calciumcarbonate, talc, kaolin, bentonite, attapulgite, diatomaceous earth,silica, colloidal silica, fumed silica, corn starch, carbon black,microcrystalline cellulose, iron oxide, aluminum oxide, calciumstearate, magnesium stearate, or combinations thereof.

In embodiments, the anticaking agent is a second outer layer formed bycrosslinking the surface of the modified proppant. The addition of aspecies capable of crosslinking the swellable polymer on the proppantsurface can effectively reduce the ability of the polymer layer to swellprematurely. Decreased swelling of the polymer will reduce the tendencyof the modified proppant to experience caking or agglomeration duringtransport and storage. In embodiments, the crosslinking species has thecapability of forming a bond with either a hydroxyl functional group, acarboxyl functional group, an amine functional group, or an amidefunctional group. The crosslinking species can be chosen from organiccompounds containing aldehyde, amine, anhydride, or epoxy functionalgroups. The crosslinking species can also be an organometallic compound.In embodiments, the crosslinking species forms a bond that can be brokenor removed under mechanical shear. Organometallic compounds able toassociate and/or bond with hydroxyl and carboxyl functional groups arean example of a crosslinking species that form shear-sensitive bonds.Upon the high shear of pumping associated with hydraulic fracturing, thecrosslink on the polymer can be degraded and the polymer is able toswell unhindered once the modified proppant is introduced into thehydraulic fracturing fluid.

In embodiments, the anticaking agent is a thin second layer of a solidnon-hygroscopic material such as fatty acids, hydrogenated fatty acids,hydrogenated oils, waxes, polyethylene, polyethylene oxides,polypropylene oxides, copolymers of polyethylene oxide and polypropyleneoxide, or combinations thereof. Examples of fatty acids suitable for useas a second layer include stearic acid, palmitic acid, lauric acid ortallow fatty acids containing stearic acid, palmitic acid/or lauricacid. Examples of hydrogenated oils suitable for use as a second layerinclude hydrogenated castor oil. Examples of waxes suitable for use as asecond layer include paraffin, petroleum jelly and slack wax. A thin,solid layer can be applied to the surface of the modified proppant tocreate a barrier preventing the swellable polymer layer on adjacentmodified proppant particles from adhering to each other during storage.The solid outer layer utilized can be comprised of compounds that areeither water soluble, water insoluble or both. The solid outer layer isnon-hygroscopic. The solid outer layer is chosen such that it remains inthe solid phase at temperatures below 38° C. and has a melting point inthe range of 40° C. to 120° C. In embodiments, the outer layer is chosensuch that the melting point is low enough that the outer layer will bein the liquid phase during the drying process in the manufacturing ofthe modified proppant yet high enough that the outer layer will exist inthe solid phase during storage and transport of the modified proppant.The solid phase outer layer acts as a barrier to prevent/reduce cakingof the modified proppant due to humid environments. The solid outerlayer can be added to the modified proppant as a finely divided powder,a flake, a solution in an oil carrier or as a warm liquid. The solidouter layer can be added to the modified proppant immediately before thepolymer, simultaneously with the polymer, as a blend with the polymer,or can be added at some time after addition of polymer but before thedrying process. Preferably, the solid outer layer anticaking agent isadded after the polymer has been well mixed with the modified proppantbut before the modified proppant is dried.

In embodiments the anticaking agent is a second layer of a saltpossessing a monovalent cationic charge that can be added to themodified proppant as a liquid or solution in oil at temperatures below100° C. such as a cationic surfactant or a monovalent salt hydrate.Cationic surfactants comprising a quaternary amine with a hydrophobictail such as commercially available Adogen 464 or Arquad 2HT-75 fromAkzo Nobel can be used as a second coating to give the modified proppanta hydrophobic layer while also neutralizing the potential anionic chargeof the polymer. Many salt hydrates, such as sodium acetate trihydrateand sodium aluminum sulfate dodecahydrate, have melting points below100° C. and can be added to the modified proppant and melted on as asecond layer during the drying process of the modified proppant. In boththe case of the cationic surfactant and the salt hydrate a concentratedlayer of cationic charge on the modified proppant surface is achievedthat can reduce the swelling potential of an anionically chargedpolymer. Upon introduction of the modified proppant to an aqueous streamthe monovalent salt would be sufficiently diluted so as to allow themodified proppant to perform as intended.

In embodiments, the anticaking agent is a hydrophobic, lubricious oilsecond layer applied to the modified proppant, selected from the groupconsisting of silicone oils, mineral oils, petroleum jellies,triglycerides or a combination thereof. Examples of silicone oilssuitable for use as a hydrophobic lubricious second layer includepolydimethylsiloxane. Examples of triglycerides suitable for use as ahydrophobic lubricious second layer include corn oil, peanut oil, castoroil and other vegetable oils. Preferably, the hydrophobic lubricious oilhas a smoke point and a boiling point above the temperature used in thedrying stage of manufacturing the modified proppant. Preferably thesmoke point of the oil is above 200° C. Preferably the smoke point ofthe oil is at least 175° C.

The hydrogel-modified proppants of the invention can advantageously usea localized polymer concentration on the proppant surface, in contrastto the traditional approach of making the entire fluid medium viscous.This localized hydrogel layer can permit a more efficient use ofpolymer, such that a lower total amount of polymer can be used tosuspend proppant, as compared, for example, with conventionalpolymer-enhanced fracturing fluids such as slickwater, linear gel, andcrosslinked gel. Although the hydrogel-modified proppants are consideredto be self-suspending, they can be used in combination with frictionreducers, linear gels, and crosslinked gels.

The hydrogel-modified proppants as disclosed herein can have theadvantage of delivering friction-reducing polymer into the fracturingfluid, so that other friction reducer polymers might not be required ormight be required in lesser amounts when the hydrogel-modified proppantsare used in hydraulic fracturing operations. In embodiments, some of thehydrogel polymer can desorb from the surface of the proppant to deliverfriction reducing benefits or viscosity features to the fracturingfluid. While the exemplary embodiments herein focus on the use ofhydrogel-modified proppants for hydraulic fracturing purposes, otheruses for hydrogel-modified proppants can be envisioned, where theircapabilities for water retention or friction reduction can be exploited.For example, hydrogel-modified proppants can be used for absorbing waterfrom moist environments, forming water-retaining particles that can beremoved from the environment, carrying with them undesirable moisture.As another example, hydrogel-modified proppants can be used insituations where adding water to an environment would be advantageous. Ahydrogel-modified proppant can be saturated with water or an aqueoussolution and then used, for example, as a soil remediation additive in adry environment. The hydrogel-modified proppant can be formed from sandor other substrates that are compatible with the soil, and they can betransported to the area of interest in dry form; they then can besaturated with water and used as a soil amendment. In other embodiments,hydrogel-modified proppants can be used as a soil amendment in dry form,where they can absorb and hold moisture from the environment, fromirrigation, from rainfall and the like. In these embodiments, themoisture-holding properties of the hydrogel-modified proppant can beused advantageously. In embodiments, the hydrogel-modified proppant canbe used to reduce erosion of topsoil, seedbeds, hydroseeding mixtures,and the like. In embodiments, the hydrogel-modified proppant can be usedas a vehicle for introducing other compatible agents into the region,for example into the soil. Hydrogel-modified proppants can compriseadditional formulations that leach out of or through the hydrogel layerinto the environment, either as the hydrogel degrades, or as it absorbsmoisture and expands. Examples of these formulations includefertilizers, seeds, plant growth regulators, herbicides, pesticides,fungicides, and the like. Other uses for hydrogel-modified proppantsprepared in accordance with these formulations and methods can beenvisioned that are consistent with their properties described herein.

The hydrogel polymer used for preparation of hydrogel-modified proppantscan, in embodiments, comprise polymers such as a polyacrylamide,copolymers of acrylamide with anionic and cationic comonomers,hydrolyzed polyacrylamide, copolymers of acrylamide with hydrophobiccomonomers, poly(acrylic acid), poly(acrylic acid) salts, carboxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, guar gum,alginate, carrageenan, locust bean gum, carboxymethyl guar,carboxymethyl hydroxypropyl guar gum, hydrophobically associatingswellable emulsion (HASE) polymers, latex polymers, starches, and thelike. In embodiments, the hydrogel polymer can have a molecular weight(g/mol) above 1 million, for example a range of 10 million to 40 millionDaltons. In embodiments, the hydrogel polymer can be a high molecularweight vinyl addition polymer that is water soluble and has a linearstructure.

In embodiments, the hydrogel polymer can be crosslinked as describedabove to enhance the water absorbing and swelling properties of thepolymer. The crosslinkers can be introduced as an element of thehydrogel base polymer, or they can be introduced as chemical modifiersfor pre-formed polymers. The crosslinking species can be added directlyinto the polymer used to coat the proppant, simultaneously added to theproppant with the polymer while mixing, or added some time afteraddition of the polymer to the proppant but before drying.

Localizing the polymer around the proppant surface as described hereincan result in a more effective use of polymer and can prevent proppantfrom settling out of a polymer solution. In embodiments, the polymerlayer hydrates around the proppant effectively preventingproppant/proppant (interparticle) contact. This can prevent the proppantfrom forming a compact settled bed and can result in a proppant that iseasier to resuspend in a fracturing fluid. The resuspension propertiesfor the modified proppants can be important if the fluid flow isinterrupted during hydraulic fracturing operations. In this event, whenthe flow is resumed it is important that the proppant can be resuspendedto avoid the loss of proppant or the unintended blockage of a fluidpath.

The polymer surface modifications as described herein can cause anincrease in the effective hydrodynamic radius of the proppant particlewhen the polymer swells. This can result in increased drag on theproppant as well as effectively changing the overall hydrogel/particledensity. Both can result in a proppant particle with a slower settlingrate and superior transport properties.

The hydrogel-modified proppants of the invention can advantageously usea localized polymer concentration on the proppant surface. Preferably,after the hydrogel is hydrated in water and exposed to shear conditionssuch as pipeline transport, much of the hydrogel polymer remainsassociated with the proppant surface. In embodiments, the manufacturingprocess of coating the substrate particles with the hydrogel polymercauses a physical or chemical attachment of the polymer onto theproppant surface. This attachment can be caused by entanglement of thepolymer chains upon drying of the hydrogel film, leading to a hydrogelcoating that resists desorption upon exposure to shear in the hydratedstate. In embodiments, the entanglement of the polymer chains is aidedby chemical reaction or interaction of the polymer chains. Inembodiments, a linear, non-crosslinked hydrogel polymer is used as acoating to enable the linear polymer chains to become entangled uponformation of the polymer coating. In embodiments, the polymerentanglement is aided by the drying process during manufacturing and bythe use of coalescing aids. The coalescing aids are additives that causethe individual emulsion droplets of the coating formulation to becomecoalesced into a continuous film upon drying. In embodiments, thecoalescing aid is an alcohol such as propanol, glycerol, propyleneglycol, or ethylene glycol. FIGS. 4A and 4B show two scanning electronmicrographs (SEM) that demonstrate the effect of glycerol in causing thecoalescence of the coating polymer into a continuous film. In thescanning electron micrograph image in FIG. 4A, the proppant was coatedwith a hydrogel formulation of anionic polyacrylamide emulsion withoutadded glycerol and then dried at 100° C. for 1 hour. In the image ofFIG. 4A, a complete coating of the proppant grain surface is seen, butthe individual emulsion droplets of approximately 1 micron diameter arestill visible. In the SEM image in FIG. 4B, the proppant was coated withthe same anionic polyacrylamide emulsion but with 10 wt. % addedglycerol as a coalescing agent; it was then dried at 100° C. for 1 hour.The effect of the coalescing agent is evident in the appearance of thedried film: the SEM image in FIG. 4B shows a substantially completecoating of the proppant grain, and in this case the emulsion dropletshave coalesced into a more continuous film. FIG. 5 shows a SEM image ofa dried hydrogel film made in the same way as the sample in FIG. 4B. Thehydrogel film in FIG. 5 shows good coalescence of the emulsion dropletsinto a film, and complete coverage of the proppant grain surface.

In embodiments, polymer pairing, optionally in combination with ioniccoupling, can be used to improve the hydrogel polymer retention on thesurface of the proppant particles. For example, a cationic polymer canbe deposited onto the proppant as a first layer to “lock in place” asecond layer containing a hydrogel such as a high molecular weightanionic polymer by ionic coupling. In embodiments, the cationic polymercan be polydiallyldimethylammonium chloride (poly-(DADMAC)), linearpolyethylenimine (LPEI), branched polyethylenimine (BPEI), chitosan,epichlorohydrin/dimethylamine polymer, ethylene dichloride dimethylaminepolymer, or cationic polyacrylamide. The cationic polymer layer can beplaced on the proppant either before or after proppant surfacemodification with the anionic hydrogel layer. The ionic couplinginteraction can act as an anchoring mechanism to help prevent theanionic polymer from desorbing in high shear environments such as goingthrough a pump or during pumping down the wellbore. The cationic polymercan also improve polymer retention by causing a delay in the hydrationand extension of the anionic polymer chains. It is believed that lesspolymer chain extension during the pumping process will yield higherpolymer retention on the proppant (i.e. less desorption).

In embodiments, covalent crosslinking of the hydrogel polymer layer onproppant surface can improve the swelling properties of the polymer andthe shear tolerance to prevent premature release of the hydrogel fromthe proppant. Covalent crosslinkers can include the following functionalgroups: epoxides, anhydrides, aldehydes, diisocyanates, carbodiamides,divinyl, or diallyl groups. Examples of these covalent crosslinkersinclude: PEG diglycidyl ether, epichlorohydrin, maleic anhydride,formaldehyde, glyoxal, glutaraldehyde, toluene diisocyanate, methylenediphenyl diisocyanate, 1-ethyl-3-(3-dimethylaminopropyl) carbodiamide,methylene bis acrylamide, and the like. Covalent crosslinking of thehydrogel polymer layer on the proppant surface can effectively create aswellable “polymer cage” around the proppant. The covalent bonds preventthe polymer from completely desorbing into solution. The slightlyinsoluble polymer layer is able to swell and produce a hydrated polymerlayer.

To further prevent the possible detachment of the hydrogel from thesurface of the particle, the proppant particle can be treated to impartfunctionalities that will also take part in the polymerization process.For example, sand particles can be treated with silanes to yieldparticles with vinyl functionalities, hydroxyl, epoxy, etc.

Delayed/controlled hydration of polymer layer may be desirable to delaythe hydration of the polymer surface modification during handling of theproppant and initial pump-down through the wellbore. Environmentalfactors such as humidity and rain could cause premature hydration of thepolymeric coating, which would make it difficult to effectively meterthe proppant dose into the blender during a hydraulic fracturingoperation. It is also believed that a fully hydrated polymer layer canbe more prone to desorption under the high shear conditions associatedwith pumping of a fracturing fluid down the tubular. For these reasons,it may be advantageous to engineer a surface-modified proppant havingslower or delayed hydration properties. In embodiments, delayedhydration can be achieved by addition of a low hydrophilic-lipophilicbalance (HLB) surfactant, exclusion of a high HLB finishing surfactant,comonomers that reduce solubility, charge shielding using a monovalentsalt, or by incorporation of a hydrophobic layer such as a fatty acid,or a fatty alcohol.

In embodiments, hydrophobic groups can be incorporated into the hydrogelpolymer to allow for hydrophobic interactions. This method can improvethe salt tolerance of the hydrogel layer, such that the hydrogel layerremains swellable even in an aqueous fluid that contains elevated saltconcentrations.

Since the goal of the hydrogel coating is to improve hydraulic transportof the proppant, it is important that the hydrated hydrogel layerremains attached or localized on the proppant surface when exposed toshear conditions during fluid transport. However, upon placement of thehydrogel coated proppant in a fractured well, the hydrogel polymershould degrade or disentangle to detach from the proppant grains andyield a proppant pack with sufficient hydraulic conductivity to enablethe production of fluids. The removal of the hydrogel layer from theproppant is caused by environmental factors such as elevatedtemperatures, microbial action, and the presence of breakers, brine,and/or hydrocarbons. In a preferred embodiment, after thehydrogel-coated proppant is pumped into a well, the hydrogel isdegraded, disentangled, dissolved, or detached with the assistance abreaker such as an oxidizer or an enzyme. The oxidizer type of breakerscan be peroxides, magnesium peroxide, calcium peroxide, a persulfatesalt, sodium bromate, sodium hypochlorite, ozone, sodium nitrate, andthe like. A blend of a first oxidant that activates at lowertemperature, such as ammonium persulfate, with a second oxidant thatactivates at higher temperature, such as magnesium peroxide, can improvethe breaking of the hydrogel after placement of the hydrogel coatedproppant. Enzyme-based breakers are known in the art and are commonlyused to break down the viscosity of fluids pumped into wells. Theenzymes promote reactions to degrade or cleave polymer linkages. In somecases enzyme breakers can provide a more efficient break because theytarget and bind the hydrogel polymer(s). Enzyme breakers are typicallymore effective at lower to moderate temperatures and can be combinedwith oxidizers that activate at higher temperatures. Selection of theappropriate enzyme breaker based on the identity of the hydrogel polymerand bottom hole conditions can improve the breaking of the hydrogel.

Also disclosed herein, is a method of fracturing a well using a hydrogelcoated proppant in combination with non-hydrogel-coated proppant. Forexample, the hydrogel-coated proppant can serve as a suspending agentfor the non-hydrogel-coated proppant.

In particular embodiments, the hydrogel polymer is selected so that itshydration is essentially complete at least by the time the modifiedproppant carrying this hydrogel polymer reaches its destination. Inembodiments, the destination in a downhole application is the area inthe well where the modified proppant enters the geological formation tobe fractured, for example where the direction of travel for thehydraulic fracturing fluid changes from vertical to horizontal, or wherethe direction of the drill string begins to change from vertical tohorizontal. The self-suspending feature of preferred modified proppantsof this disclosure can be particularly exploited in hydraulic fracturingfluids that are moving in a generally horizontal direction. Inembodiments, the hydrogel polymer for the modified proppant is selectedso that its hydration is essentially complete within 2 hours, within 1hour, within 40 minutes, within 30 minutes, within 20 minutes or evenwithin 10 minutes of being contacted with an excess of tap water at 20°C.

Also disclosed herein is a method of improving well productivity byimproved proppant placement using a hydrogel-coated proppant. Thehydrogel-coated proppant can be more effectively transported into thefar end of fractures to enable higher productivity of oil and gas from awell. Because the surface-modified proppants disclosed herein can beless inclined to settle out of the fluid and easier to resuspend andtransport through the fracture, it is believed that proppant placementwill be more effective. The ability to transport proppant further intofractures could significantly increase the effectiveness of a fracturingstimulation operation, resulting in a larger of volume of higher densityfractures. These fracture channels can advantageously allowgas/condensate to more easily flow into the wellbore from the reservoir.

Also disclosed herein is an improved method of proppant placement usinga low viscosity fluid. The surface modified proppants as disclosedherein utilize polymers more effectively to suspend/transport proppantparticles. The surface modification renders the proppantself-suspending, thereby reducing or eliminating the need for highlyviscous fluids/gels to transport proppant. Hence, lower viscosity fluidscan be used in combination with the surface-modified proppant totransport proppant into fractures. This would advantageously simplifythe formulation of fracturing gels for use with proppants.

Also disclosed herein is a more efficient method of fracturing a wellusing less proppant. Because highly effective proppant placement can beachieved with the easily-transportable surface-modified proppants asdisclosed herein, it is anticipated that a smaller amount of thesesurface-modified proppants would be required for any given fracturingoperation, as compared to systems using traditional proppants. With anincreasing demand for fracturing grade sand/proppants, and a decreasingsupply of desirably-shaped sand for proppant use, it would beadvantageous to provide systems and methods such as those disclosedherein where less proppant can be used to achieve results comparable toor superior to the outcomes using current techniques.

After the hydrogel coated proppants of the invention have been pumpedinto a well, the hydrogel layer can be degraded by chemical, thermal,mechanical, and biological mechanisms. Specifically, the polymericsurface modification on the proppant can be broken down with the aid ofchemical breakers, for example, ammonium persulfate, magnesium peroxide,or other oxidizers. The polymeric surface modification on the proppantcan also be broken down with the aid of ambient reservoir conditions,such as elevated brine content, elevated temperature, and contact withhydrocarbons. Controlled breaking of the hydrogel layer upon reaching atarget temperature or amount of time in the fluid can be used as a meansto direct the placement of the proppant in the desired location infractures. The degradation of the hydrogel layer is also beneficial toensuring the adequate conductivity of the propped fracture aftercompleting the hydraulic fracturing operations. In embodiments, thehydrogel layer can demonstrate stimuli-responsive properties, so that itswells with water when exposed to a first set of conditions, such as acertain first temperature or pH, and it loses water, loses volume, losesthickness, or even collapses, when subjected to a certain set ofconditions, such as a second temperature or pH.

For example, in an embodiment, temperature-responsive hydrogels can becoated onto proppant materials. The temperature responsive hydrogellayer can swell when exposed to water at a first set of conditions, suchas a water temperature of 50-100 degrees F., and then it can collapsewhen exposed to a second set of conditions, such as a water temperatureof 110-450 degrees F. Using this stimuli-responsive mechanism, thetemperature responsive hydrogel coated proppant can have self-suspendingproperties as the fracturing fluid carries it underground to thelocation of the fractures at an initial water temperature, for example50-100 degrees F. As the coated proppant encounters the highertemperature region of the underground formation, such as 110-450 degreesF., the hydrogel layer can collapse, allowing deposition andconsolidation of the proppant in the fissures. The temperatureresponsive hydrogel can be a water soluble polymer or copolymercomposition comprising hydrophobic monomers selected from the groupconsisting of alkyl acrylate esters, N-alkyl acrylamides, propyleneoxide, styrene, and vinylcaprolactam. The N-alkyl substitutedacrylamides can be N-isopropylacrylamide, N-butylacrylamide,N-octylacrylamide, and the like. The alkyl acrylate esters can besubstituted by alkyl chains having from 1 to about 30 carbons. In apreferred embodiment, the temperature responsive hydrogel polymercomprises N-isopropylacrylamide and contains up to about 90 percent ofhydrophilic comonomer units. The type and amount of the hydrophobicmonomer substituent in the hydrogel polymer can be selected byexperimental optimization techniques to adjust the water solubility andthe temperature responsive properties of the hydrogel polymer.

Also disclosed herein is a method of delivery of additives, for example,chemical additives, into the proppant pack, by incorporating theadditives into the hydrogel layer of the modified proppant. Theadditives can include chemical additives that can be advantageouslydelivered in the hydrogel layer, for example scale inhibitor, biocide,breaker, wax control, asphaltene control, and tracers. The chemicaladditives can be in the form of water soluble materials, water insolubleparticles, fibers, metallic powders or flakes, and the like. Thechemical additives can be selected such that they slowly dissolve ordecompose to release their chemical activity.

In embodiments, chemical additives can be chemically bound to thepolymer in the hydrogel layer, for example, by covalent bonding, ionicbonding, hydrophobic association, hydrogen bonding, or encapsulation.The chemical additives can be added to the proppant separately from thehydrogel, or they can be combined with the hydrogel coating formulationat the time of manufacture of the coated proppant. Breaker chemicalssuch as persulfates, peroxides, permanganates, perchlorates, periodatesor percarbonates can be added in this method of delivery. The transportand delivery of these chemicals with the hydrogel coated proppant hasthe advantage of a targeted delivery of the chemicals to a fracture orto a proppant pack. This method offers the advantage of concentratingthe chemical additives in the location where their function is needed,thereby delivering the chemical additive more efficiently, moreeffectively, and at lower concentration. In embodiments, the desorption,oxidation, or degradation of the hydrogel polymer can result in thecontrolled release of the chemical additives from the self-suspendingproppant.

In embodiments, a hydraulic fracturing operation can have multiplestages of fracturing; the proppants injected in each stage can containunique chemical additives acting as tracers. Tracers are commonly usedin hydraulic fracturing, including tracers that can be detected by highperformance liquid chromatography (HPLC), gas chromatography (GC),ultraviolet or visible absorbance, and radioactive signal measurement.Analysis of the fluids produced from the fractured well can provideinformation about the relative productivity of each fracturing stage bythe presence and concentration of the unique tracers corresponding tothe stages. In other embodiments, additives acting as breakers can becarried in the hydrogel layer, e.g., by physical binding or entanglementin the polymer layer. In embodiments, the breakers can be modified witha cationic surface coating to provide an anchoring mechanism to attachthe breakers to the anionic hydrogel of the self-suspending proppant.For example, magnesium peroxide powder can be coated with a cationicpolymer such as poly-DADMAC, and this cationically modified magnesiumperoxide can be blended with the hydrogel coated proppant either beforeor after the proppant is introduced into the hydraulic fracturing waterstream. Using this approach, the breakers are transported to the samelocation as the hydrogel proppants, so the breaker can be efficientlytargeted at the hydrogel layer. The oxidative breakers can have anaccelerated activity at higher temperatures. Using this method, thebreaker chemicals incorporated in the hydrogel layer can becomeactivated upon placement in the fractures, for example, by the elevatedtemperatures of the petroleum bearing reservoir.

In other embodiments, breakers can be pumped into a subterraneanformation before and/or after the introduction of the hydrogel-coatedproppants. In the case that the breaker is pumped before the proppant,the fluid containing the excess breaker will flow back through theproppant pack and have the ability to assist degradation of the hydrogellayer after the proppant has reached its destination. In the event thatbreaker is pumped into the propped formation after the proppant, thebreaker can infiltrate the proppant pack and have its effect on thebreakdown of the hydrogel layer. In embodiments, the breaker can beadded at multiple times to assist breaking the hydrogel layer. Inembodiments, the breakers can be used in a combination of types, forexample, a lower temperature activated breaker such as ammoniumpersulfate can be used for a quick effect, in combination with anencapsulated, longer acting, or higher temperature activated breakersuch as magnesium peroxide, to give a sustained effect of breaking thehydrogel layer over the course of time before the fluids are flowed backand the well is put into production.

In embodiments, the surface of a proppant particulate substrate can becoated with a selected polymer, either as a single layer or as a seriesof multiple coating layers. The coating (either single layer ormultilayer) can show switchable behavior under certain circumstances. Asused herein, the term “switchable behavior” or “switching behavior”refers to a change in properties with a change in circumstances, forexample, a change from one set of properties during the transport phaseand another set of properties inside the fracture. Switching behaviorcan be seen, for example, when a particle demonstrates hydrophilicproperties in the fracturing fluid and adhesive properties when in placewithin the fractures. Such behavior can triggered by circumstances likethe high closing pressures inside the fracture site so that the outerlayer of the coating rearranges itself to exhibit more advantageousproperties.

In an embodiment, the coated particle can switch from hydrophilic tohydrophobic when subjected to the high pressures inside the fractures.In an exemplary embodiment, during the transport phase, when thehydrophilic covering of the particle is exposed to the water-basedfracturing fluid, it will tend to be fully distended. As a result, thecoating can provide the particle with lubrication in this state,facilitating its movement through the proppant slurry. When the particlehas been conveyed to its destination within the fractures in theformation though, the high pressures there will overcome the stericrepulsions of the external hydrophilic polymer chains, forcing the outerlayer to rearrange itself so that the inner layer is exposed. Inembodiments, the switchable inner layer can be hydrophobic or adhesive,or both. As the inner layer becomes exposed, its properties can manifestthemselves. If the inner layer has adhesive properties, for example, itcan fix the particles to each other to prevent their flowback. Thisinner layer can also be configured to capture fines in case the proppantparticle fails. Moreover, the residual intact hydrophilic groups presentin the outer coating can allow easy flow of oil through the proppantpack.

In embodiments, a coated proppant particle can be produced that bearsthe following layers of coating. First, a pressure-activated fixativepolymer can be used to coat the proppant substrate. This coating layercan be elastomeric, thereby providing strength to the proppant pack byhelping to agglomerate the proppant particles and distribute stress. Inaddition, this coating layer can encapsulate the substrate particles andretain any fines produced in the event of substrate failure. Second, ablock copolymer can be adsorbed or otherwise disposed upon the firstlayer of coating. The copolymer can have a section with high affinityfor the first polymeric layer, allowing strong interaction (hydrophobicinteraction), and can have another section that is hydrophilic, allowingfor easy transport of the proppant in the transport fluid.

In certain embodiments, a stronger interaction between the first andsecond coating layers may be useful. To accomplish this, aswelling-deswelling technique can be implemented. For example, the blockcopolymer can be adsorbed onto the surface of the elastomeric-coatedparticle. Then, the first coating layer can be swelled with smallamounts of an organic solvent that allow the hydrophobic block of thecopolymer to penetrate deeper into the first coating layer and to becomeentangled in the elastomeric coating. By removing the organic solvent,the layered polymeric composite will deswell, resulting in a strongerinteraction of copolymer with the elastomeric particle. A method forswelling-deswelling technique that can be useful is set forth in“Swelling-Based Method for Preparing Stable, Functionalized PolymerColloids,” A. Kim et al., J. Am. Chem. Soc. (2005) 127: 1592-1593, thecontents of which are incorporated by reference herein.

In embodiments, proppant systems using coatings as disclosed herein candecrease the amount of airborne particles associated with proppantmanufacture. For example, respirable dust including fine crystallinesilica dust associated with handling and processing proppant sand can becaptured and held by the proppant coatings during their processing. Inembodiments, coating agents can be added that have a particular affinityfor particulates in the environment that could adversely affect workersafety or create nuisance dust problems. In embodiments, a hydrogelcoating on proppant particles can serve as a binder or capturing agentby mechanically entrapping or adhering to the dust particulates.

While the systems described herein refer to a two-layer coating system,it is understood that there can be multiple (i.e., more than two)coating layers forming the composite proppant particles disclosedherein, with the each of the multiple coating layers possessing some orall of the attributes of the two coating layers described above, or withone or more of the multiple coating layers providing additionalproperties or features.

The addition of a species capable of crosslinking the swellable polymeron the proppant surface can effectively reduce the ability of thepolymer layer to swell prematurely. Decreased swelling of the polymercan reduce the tendency of the polymer-coated proppant to undergo cakingduring storage in humid conditions. In some embodiments, the crosslinkerwill not impede hydration/swelling of the polymer coating once thepolymer-coated proppant is dispersed in an aqueous fluid, such as ahydraulic fracturing fluid. In embodiments, the crosslinking species hasthe capability of forming a bond with a carboxyl functional group, anamide functional group, or both. In certain aspects, the crosslinkingspecies forms a bond that can be broken or removed under mechanicalshear or by the action of a chemical breaker. The crosslinking speciescan be added directly into the polymer used to coat the proppant,simultaneously added to the proppant with the polymer while mixing, oradded some time after addition of the polymer to the proppant but beforedrying.

The crosslinking species can be chosen from organic compounds containingaldehyde, amine, anhydride, or epoxy functional groups. The crosslinkingspecies can also be an organometallic compound. Organometallic compoundsable to associate and/or bond with the carboxyl functional group are anexample of a crosslinking species that form shear sensitive bonds. Insuch embodiments, the organometallic compound is able to reduce theswelling tendency of the polymer-coated proppant via crosslinking thecarboxyl groups prior to the introduction of the proppant into ahydraulic fracturing fluid. Then, when the crosslinked polymer coatingencounters the high shear forces of pumping associated with hydraulicfracturing, the crosslink on the polymer can be degraded, allowing thepolymer to swell unhindered when the proppant is introduced into thehydraulic fracturing fluid.

In certain embodiments, a thin, non-hygroscopic coating layer can beapplied to the surface of a hydrogel-coated proppant to create a barrierpreventing the swellable polymer layer on adjacent proppant particlesfrom adhering during storage. The outer layer utilized can be comprisedof compounds that are water-soluble, water insoluble or both. Inembodiments, the outer layer can be formulated such that it remains inthe solid phase at temperatures below 40° C. and has a melting point inthe range of 40° C. to 120° C. Preferably, the outer layer is formulatedsuch that the melting point is low enough that the outer layer will bein the liquid phase during the drying process in the manufacturing ofthe polymer coated proppant, yet is high enough that the outer layerwill exist in the solid phase during storage and transport of thepolymer coated proppant.

In these embodiments, the outer layer acts as a barrier to reduce cakingof the coated proppant in humid environments. As used herein, the term“caking” refers to the formation of clumps or solid masses by adhesionof the loose granular material. Caking of proppants during storage isundesirable for material handling purposes. The hydrophobic outer layercan be added to the polymer-coated proppant as a finely divided powderor as a liquid. In embodiments, the outer layer material can be meltedprior to addition to the coated proppant; in other embodiments the outerlayer material can be added as a solid or waxy material, which can meltduring the drying process. The solid outer layer can be added to theproppant simultaneously with the polymer or can be added at some timeafter addition of polymer but before the drying process. The outer layercan be comprised of fatty acids, hydrogenated oils, vegetable oils,castor oil, waxes, polyethylene oxides, polypropylene oxides, and thelike.

2. Particulate Substrate Materials

Composite proppant particles in accordance with these systems andmethods can be formed using a wide variety of proppant substrateparticles. Proppant particulate substrates can include for use in thepresent invention include graded sand, resin coated sand, bauxite,ceramic materials, glass materials, walnut hulls, polymeric materials,resinous materials, rubber materials, and the like, and combinationsthereof. The self-suspending proppant (“SSP”) as disclosed herein canalso be made using specialty proppants, such as ceramics, bauxite, andresin coated sand. By combining sand SSP with specialty SSP, a proppantinjection can have favorable strength, permeability, suspension, andtransport properties. In embodiments, the substrates can includenaturally occurring materials, for example nutshells that have beenchipped, ground, pulverized or crushed to a suitable size (e.g., walnut,pecan, coconut, almond, ivory nut, brazil nut, and the like), or forexample seed shells or fruit pits that have been chipped, ground,pulverized or crushed to a suitable size (e.g., plum, olive, peach,cherry, apricot, etc.), or for example chipped, ground, pulverized orcrushed materials from other plants such as corn cobs. In embodiments,the substrates can be derived from wood or processed wood, including butnot limited to woods such as oak, hickory, walnut, mahogany, poplar, andthe like. In embodiments, aggregates can be formed, using an inorganicmaterial joined or bonded to an organic material. Desirably, theproppant particulate substrates will be comprised of particles (whetherindividual substances or aggregates of two or more substances) having asize in the order of mesh size 4 to 100 (US Standard Sieve numbers). Asused herein, the term “particulate” includes all known shapes ofmaterials without limitation, such as spherical materials, elongatematerials, polygonal materials, fibrous materials, irregular materials,and any mixture thereof.

In embodiments, the particulate substrate can be formed as a compositefrom a binder and a filler material. Suitable filler materials caninclude inorganic materials such as solid glass, glass microspheres, flyash, silica, alumina, fumed carbon, carbon black, graphite, mica, boron,zirconia, talc, kaolin, titanium dioxide, calcium silicate, and thelike. In certain embodiments, the proppant particulate substrate can bereinforced to increase their resistance to the high pressure of theformation which could otherwise crush or deform them. Reinforcingmaterials can be selected from those materials that are able to addstructural strength to the proppant particulate substrate, for example,high strength particles such as ceramic, metal, glass, sand, and thelike, or any other materials capable of being combined with aparticulate substrate to provide it with additional strength.

In addition to bare or uncoated substrates, composite hydrogel-coatedproppants can be formed from substrates that have undergone previoustreatments or coatings. For example, a variety of resin-coated proppantparticles are familiar to skilled artisans. The formulations and methodsdescribed above for coating are suitable for use with coated or treatedproppant particles, including curable and precured resin coatedproppants.

In one embodiment for treating resin-coated sand, a swellable hydrogellayer, as described above, can be applied to the resin-coated sand toimprove its suspension characteristics. In embodiments, one can includethe addition of the species that acts as an adhesion promoter to attachthe hydrogel to the resin layer. The adhesion promoters can be, forexample, block co-polymers composed of both hydrophilic and hydrophobicmonomers. The block co-polymer can be added after the substrate sand isresin-coated or at the same time as the resin coating. In addition toblock co-polymers, cationic species can be used such as fatty amines,polyquaternary amines, and cationic surfactants.

In certain embodiments, the proppant particulate substrate can befabricated as an aggregate of two or more different materials providingdifferent properties. For example, a core particulate substrate havinghigh compression strength can be combined with a buoyant material havinga lower density than the high-compression-strength material. Thecombination of these two materials as an aggregate can provide a coreparticle having an appropriate amount of strength, while having arelatively lower density. As a lower density particle, it can besuspended adequately in a less viscous fracturing fluid, allowing thefracturing fluid to be pumped more easily, and allowing more dispersionof the proppants within the formation as they are propelled by the lessviscous fluid into more distal regions. High density materials used asproppant particulate substrates, such as sand, ceramics, bauxite, andthe like, can be combined with lower density materials such as hollowglass particles, other hollow core particles, certain polymericmaterials, and naturally-occurring materials (nut shells, seed shells,fruit pits, woods, or other naturally occurring materials that have beenchipped, ground, pulverized or crushed), yielding a less dense aggregatethat still possesses adequate compression strength.

Aggregates suitable for use as proppant particulate substrates can beformed using techniques to attach the two components to each other. Asone preparation method, a proppant particulate substrate can be mixedwith the buoyant material having a particle size similar to the size ofthe proppant particulate substrates. The two types of particles can thenbe mixed together and bound by an adhesive, such as a wax, aphenol-formaldehyde novolac resin, etc., so that a population of doubletaggregate particles are formed, one subpopulation having a proppantparticulate substrate attached to another similar particle, onesubpopulation having a proppant particulate substrate attached to abuoyant particle, and one subpopulation having a buoyant particleattached to another buoyant particle. The three subpopulations could beseparated by their difference in density: the first subpopulation wouldsink in water, the second subpopulation would remain suspended in theliquid, and the third subpopulation would float.

In other embodiments, a proppant particulate substrate can be engineeredso that it is less dense by covering the surface of the particulatesubstrate with a foamy material. The thickness of the foamy material canbe designed to yield a composite that is effectively neutrally buoyant.To produce such a coated proppant particulate, a particle having adesirable compression strength can be coated with one reactant for afoaming reaction, followed by exposure to the other reactant. With thetriggering of foam formation, a foam-coated proppant particulate will beproduced.

As an example, a water-blown polyurethane foam can be used to provide acoating around the particles that would lower the overall particledensity. To make such a coated particle, the particle can be initiallycoated with Reactant A, for example, a mixture of one or more polyolswith a suitable catalyst (e.g., an amine). This particle can then beexposed to Reactant B containing a diisocyanate. The final foam willform on the particle, for example when it is treated with steam whilebeing shaken; the agitation will prevent the particles fromagglomerating as the foam forms on their surfaces.

In embodiments, fibers, including biodegradable fibers can be added tothe fracture fluid along with SSP. Fibers, including biodegradablefibers, can form a fiber network that help carry the proppant with thefluid. A number of fiber types are familiar to skilled artisans foradding to fracture fluid. As would be understood by skilled artisans,fibers added to the fracture fluid can degrade under downholeconditions, and channels are formed in the proppant pack. The channelsthen have higher permeability and are therefore the flow channelsthrough which hydrocarbons travel from the formation to the wellbore.

The term “fiber” can refer to a synthetic fiber or a natural fiber. Asused herein, the term “synthetic fibers” include fibers or microfibersthat are manufactured in whole or in part. Synthetic fibers includeartificial fibers, where a natural precursor material is modified toform a fiber. For example, cellulose (derived from natural materials)can be formed into an artificial fiber such as Rayon or Lyocell.Cellulose can also be modified to produce cellulose acetate fibers.These artificial fibers are examples of synthetic fibers. Syntheticfibers can be formed from synthetic materials that are inorganic ororganic. Exemplary synthetic fibers can be formed from materials such assubstituted or unsubstituted lactides, glycolides, polylactic acid,polyglycolic acid, or copolymers thereof. Other materials to form fibersinclude polymers of glycolic acid or copolymers formed therewith, as arefamiliar to skilled artisans.

As used herein, the term “natural fiber” refers to a fiber or amicrofiber derived from a natural source without artificialmodification. Natural fibers include vegetable-derived fibers,animal-derived fibers and mineral-derived fibers. Vegetable-derivedfibers can be predominately cellulosic, e.g., cotton, jute, flax, hemp,sisal, ramie, and the like. Vegetable-derived fibers can include fibersderived from seeds or seed cases, such as cotton or kapok.Vegetable-derived fibers can include fibers derived from leaves, such assisal and agave. Vegetable-derived fibers can include fibers derivedfrom the skin or bast surrounding the stem of a plant, such as flax,jute, kenaf, hemp, ramie, rattan, soybean fibers, vine fibers, jute,kenaf, industrial hemp, ramie, rattan, soybean fiber, and banana fibers.Vegetable-derived fibers can include fibers derived from the fruit of aplant, such as coconut fibers. Vegetable-derived fibers can includefibers derived from the stalk of a plant, such as wheat, rice, barley,bamboo, and grass. Vegetable-derived fibers can include wood fibers.Animal-derived fibers typically comprise proteins, e.g., wool, silk,mohair, and the like. Animal-derived fibers can be derived from animalhair, e.g., sheep's wool, goat hair, alpaca hair, horse hair, etc.Animal-derived fibers can be derived from animal body parts, e.g.,catgut, sinew, etc. Animal-derived fibers can be collected from thedried saliva or other excretions of insects or their cocoons, e.g., silkobtained from silk worm cocoons. Animal-derived fibers can be derivedfrom feathers of birds. Mineral-derived natural fibers are obtained fromminerals. Mineral-derived fibers can be derived from asbestos.Mineral-derived fibers can be a glass or ceramic fiber, e.g., glass woolfibers, quartz fibers, aluminum oxide, silicon carbide, boron carbide,and the like.

Fibers may advantageously be selected or formed so that they degrade atspecified pH or temperatures, or to degrade over time, and/or to havechemical compatibilities with specified carrier fluids used in proppanttransport. Useful synthetic fibers can be made, for example, from solidcyclic dimers or solid polymers of organic acids known to hydrolyzeunder specific or tunable conditions of pH, temperature, time, and thelike. Advantageously, fibers can decompose in the locations to whichthey have been transported under predetermined conditioned.Advantageously, the decomposition of the fibers can yield decompositionproducts that are environmentally benign.

EXAMPLES

Materials

-   -   30/70 mesh frac sand    -   30/50 mesh frac sand    -   40/70 mesh frac sand    -   Polydiallyldimethylammonium chloride (Aldrich, St. Louis, Mo.)    -   LPEI 500 (Polymer Chemistry Innovations, Tucson, Ariz.)    -   Ethyl Alcohol, 200 Proof (Aldrich, St. Louis, Mo.)    -   Hexane (VWR, Radnor, Pa.)    -   FLOPAM EM533 (SNF)    -   Polyethyleneglycol diglycidyl ether (Aldrich, St. Louis, Mo.)    -   Glyoxal, 40 wt % solution (Aldrich, St. Louis, Mo.)    -   HFC-44 (Polymer Ventures, Charleston, S.C.)    -   Carboxymethyl Cellulose, sodium salt (Sigma-Aldrich, St. Louis,        Mo.)    -   Ammonium Persulfate (Sigma-Aldrich, St. Louis, Mo.)    -   Ethoxylated lauryl alcohol surfactant (Ethal LA-12/80%)) (Ethox        Chemical Co, SC)    -   Glycerol (US Glycerin, Kalamazoo, Mich.)    -   Potassium Chloride (Morton Salt, Chicago, Ill.)    -   Fumed Silica (Cabot, Boston, Mass.)

Example 1 Preparation of Inner Polymer Layer

An inner polymer layer of 100 ppm concentration was prepared on a sandsample by adding 200 g 30/70 mesh frac sand to a FlackTek Max 100 longjar. To the sand was added 85 g tap water and 2 g of a 1%polydiallyldimethylammonium chloride (PDAC) solution. The sample wasthen shaken by hand for approximately 5 minutes, vacuum filtered anddried in an oven at 80° C. The sand sample was then removed from theoven and used in subsequent testing.

An identical method was used as described above to formulate a 10 ppminner polymer layer coating with the exception being that only 0.2 g ofa 1% PDAC solution were used.

An identical method was used as described above to formulate an innerpolymer layer at a maximum polymer loading (“Max PDAC”) with theexception that 1 g of a 20 wt % PDAC solution was used. Followingtreatment the sand was washed with excess tap water, vacuum filtered anddried in an oven at 80° C. The sand sample was then removed from theoven and used in subsequent testing.

Example 2 Preparation of Inner Polymer Layer

An inner polymer layer of 100 ppm concentration was prepared on a sandsample by dissolving 0.2 g LPEI 500 in 10 g ethanol to form a 2% LPEI500 solution in ethanol. To 70 g ethanol in a 250 mL round bottom flaskwas added 0.75 g of the 2% LPEI 500 solution. Then 150 g of 30/70 meshfrac sand was added to the round bottom flask. The solvent was removedusing a rotary evaporator with a 65° C. water bath. The sample was thenremoved from the flask and used in subsequent testing.

Example 3 Preparation of Outer Polymer Layer

Outer polymer layers were applied to sand samples by mixing sand withliquid Flopam EM533 polymer under different conditions. In one coatingmethod, polymer product was added neat. In another coating method thepolymer product was extended by diluting with hexane. For hexanedilution 10 g polymer was added to 10 g hexane in a 40 mL glass vial andvortex mixed until homogenous. Polymer was then added to 30/70 mesh fracsand samples of 30 g in FlackTek Max 100 jars. Samples were placed in aFlackTek DAC150 SpeedMixer at 2600 rpm for about 25 seconds. Sampleswere removed from SpeedMixer and allowed to dry in an oven at 80° C.overnight.

Example 4 Performance of Outer Polymer Layer, Settling Times

Sand samples prepared in previous example were assessed for performancein a settling test. Prior to testing, all sand samples were sievedthrough a 25 mesh screen. Settling times were obtained by adding 1 g ofsand sample to 100 mL of tap water in a 100 mL graduated cylinder. Thegraduated cylinder was then inverted about 8 times and then the timerequired for all of the sand to settle at the bottom of the graduatedcylinder was recorded. Three times were recorded for each sample.Settling times are reported in Table 1.

TABLE 1 Settling Times Outer Settling Settling Settling Inner LayerTreatment Time 1 Time 2 Time 3 Sample Layer Treatment Added (g) (sec)(sec) (sec) 1 100 ppm Flopam 1 34 35 32 PDAC EM533 2 100 ppm 50:50 2 2525 26 PDAC Flopam EM533/ hexane 3 100 ppm Flopam 3 35 71 60 PDAC EM533 4100 ppm 50:50 6 24 33 32 PDAC Flopam EM533/ hexane 5 Max Flopam 1 19 2127 PDAC EM533 6 Max 50:50 2 17 23 21 PDAC Flopam EM533/ hexane 7 MaxFlopam 3 29 31 35 PDAC EM533 8 Max 50:50 6 23 24 25 PDAC Flopam EM533/hexane 9 None Flopam 1 22 22 22 EM533 10 None Flopam 3 25 54 64 EM533 11None None 0 10 10 10

Example 5 Performance of Outer Polymer Layer, Settled Bed Height

Sand samples prepared in Example 3 with outer polymer layer were alsoassessed by observing the settled bed height in water. In a 20 mL glassvial, 1 g of a sand sample was added to 10 g tap water. The vials wereinverted about 10 times to adequately wet the sand treatments. The vialswere then allowed to sit undisturbed for about 30 minutes. A digitalcaliper was then used to record the height of the sand bed in the vial.Results are reported in Table 2.

TABLE 2 Settled Bed Heights Sample 1 2 3 4 5 6 7 8 9 10 11 Bed 13.5 6.922.6 8.9 8.9 5.8 11.9 n/a 11.9 22.9 0.8 Height (mm)

Example 6 Ionic Crosslink of Outer Polymer Layer

A 40 g 30/70 mesh frac sand sample was treated with an outer polymerlayer by adding 1.3 g Flopam EM533 polymer to the 40 g of sand in aFlackTek Max 100 jar and shaking the jar by hand for 2 minutes. The sandwas then sieved through a 25 mesh screen. To assess polymer retention onsand under shear, tests were conducted by adding 10 g of treated sand to200 g tap water with different levels of PDAC in a 300 mL glass beaker.It is believed that the PDAC will interact ionically to stabilize thepolymer layer on the sand. The slurries were then stirred at 900 rpmwith an overhead mixer using a flat propeller style mixing blade for 5minutes. Mixing was then stopped and samples were allowed to settle for10 minutes. Viscosity of the supernatant was then measured using aBrookfield DV-III+ rheometer with an LV-II spindle at 60 rpm. Bed heightof the settled sand in the beaker was also recorded using a digitalcaliper. Results are reported in Table 3.

TABLE 3 Polymer Retention Sample PDAC Conc. (ppm) Visc. (cP) Bed Height(mm) 12 0 25 4.5 13 60 10 8.6 14 200 2.5 6.3

Example 7 Covalent Crosslink of Outer Polymer Layer—PEGDGE

Four samples of 30/70 mesh frac sand were treated with Flopam EM533 byadding 0.66 g polymer to 20 g sand in a FlackTek Max 100 jar and shakingby hand for 2 minutes. Then various amounts of a fresh 1%polyethyleneglycol diglycidyl ether solution in deionized water wereadded to the treated sand samples. The samples were again shaken by handfor 2 minutes and then placed in an oven at 100° C. for 1 hour. Sampleswere then removed from the oven and sieved through a 25 mesh screen. Bedheights were measured for the four samples by adding 1 g of the sandsample to 10 g of tap water in a 20 mL glass vial, inverting the vialsapproximately 10 times to adequately wet the sand and allowing the vialsto sit undisturbed for about 10 minutes. Bed heights were then measuredwith a digital caliper. Results are listed in Table 4.

TABLE 4 PEGDGE Treated Outer Polymer Layer Sample 1% PEGDGE (g) BedHeight (mm) 15 0.1 9.3 16 0.2 8.8 17 1.0 6.2 18 0 12.7

Example 8 Covalent Crosslink of Outer Polymer Layer—Glyoxal

Four samples of 30/70 mesh frac sand were treated with Flopam EM533 byadding 0.66 g polymer to 20 g sand in a FlackTek Max 100 jar and shakingby hand for 2 minutes. A 1% glyoxal solution in ethanol was formulatedby adding 0.25 g 40 wt % glyoxal to a 20 mL glass vial and diluting to10 g with ethanol. Then varying amounts of the 1% glyoxal solution wereadded to the treated sand samples, and the samples were shaken by handfor 2 minutes and placed in the oven at 100° C. for 30 minutes. The sandsamples were removed from the oven and sieved through a 25 mesh screen.For settled bed height measurements 1 g of sand was added to 10 g tapwater in 20 mL vials, inverted about 10 times and given about 10 minutesto settle. Bed height was measured with a digital caliper. Results arelisted in Table 5.

TABLE 5 Glyoxal Treated Outer Polymer Layer Sample 1% glyoxal (g) BedHeight (mm) 19 0.2 3.8 20 0.5 3.5 21 1.0 2.7 22 2.0 2.7

Example 9 Cationic/Anionic Polymer Treatments

Three samples of 30 g of 30/70 mesh frac sand were treated with PolymerVentures HCF-44 in a FlackTek Max 100 jar. The jar was shaken by handfor 2 minutes. Flopam EM533 was then added to each of the samples. Thejars were again shaken by hand for 2 minutes. The samples were thendried at 80° C. overnight. The sand samples were removed from the ovenand sieved through a 25 mesh screen. For settled bed height measurements1 g of sand was added to 10 g tap water in 20 mL vials, inverted about10 times and given about 10 minutes to settle. Bed height was measuredwith a digital caliper. Results are given in Table 6.

TABLE 6 Cationic/Anionic polymer treatment Sample HCF-44 (g) FlopamEM533 (g) Bed Height (mm) 23 0 0.45 10.26 24 0.07 0.38 8.08 25 1.0 0.355.08 26 1.5 0.30 3.94

Example 10 Sand Coated with Macromolecular Particles

A 30 g sample of 30/70 mesh frac sand was added to a FlackTek Max 100jar. To the sand, 0.3 g of paraffin wax was added. The sample was placedin a FlackTek DAC 150 SpeedMixer and mixed at 2500 rpm for 2 minutes.After mixing, 1 g of carboxymethyl cellulose was added to the sample.The sample was again placed in the FlackTek DAC 150 SpeedMixer and mixedat 2500 rpm for 1 minute. The sand sample was sieved through a 25 meshscreen. For settled bed height measurements 1 g of sand was added to 10g tap water in a 20 mL vial, inverted about 10 times and given about 10minutes to settle. The sand in this sample clumped together immediatelyand did not disperse in the water, and an accurate measurement of bedheight could not be obtained.

Example 11 Modified Sand Beaker Testing

A 30 g sample of 30/70 mesh frac sand was added to a FlackTek Max 100jar. The sand was treated with Flopam EM533 by adding 0.45 g of thepolymer to the jar and shaking by hand for 2 minutes. The sample wasthen dried at 80° C. overnight. After drying, the sample was removedfrom the oven and sieved over a 25 mesh screen. After sieving, foursamples were prepared by adding 1 g of the treated sand to 10 g of tapwater in a 20 mL vial. The vials were inverted about 10 times and leftto settle for 10 minutes. A 10% ammonium persulfate solution was made byadding 2 g of ammonium persulfate to 18 g of tap water. Varying amountsof the 10% ammonium persulfate solution were then added to the samplevials. The samples were inverted several times to mix, and then placedin an oven at 80° C. for 1 hr. After 1 hour the samples were removed andthe settled bed heights were observed. Table 7 shows the results.

TABLE 7 Breaker testing Sample 10% APS (μL) Sand Suspension 27 0Suspended 28 180 Settled 29 90 Settled 30 18 Settled

Example 12 Emulsion Additives

To determine the effect of emulsion additives on self-suspendingproppant (“SSP”) performance, we added glycerol and Ethal LA-12/80% tothe emulsion polymer EM533 before coating the proppant sand. Threedifferent polymer samples were made as follows:

-   -   SSP Polymer: 10 g of EM533, no additive    -   SSP+glycerol: 9 g EM533 and 1 g of glycerol    -   SSP+glycerol+Ethal: 9 g EM533+0.9 g glycerol+0.1 g Ethal        LA-12/80%.

Each of the above samples was vortex mixed for 30 seconds to ensurehomogeneity. To make the modified proppant, 50 g of 40/70 sand wascombined with 1.5 g of one of the polymer samples above and then mixedfor 30 s. The modified proppant samples were evaluated for shearstability in the 1 liter shear test. This test involves addition of 50grams of modified proppant to 1 liter of water in a square plasticbeaker, followed by mixing on a paddle/jar mixer (EC Engineering modelCLM-4) at 200 rpm (corresponding to a shear rate of about 550 s⁻¹) fordifferent amounts of time. The sheared samples are then poured into a1000 mL graduated cylinder and allowed to settle by gravity for 10minutes, then the bed height of the settled proppant sand is recorded.For comparison, an unmodified proppant sand will produce a bed height of10 mm after any amount of mixing. The self-suspending proppant sampleswill produce a higher bed level vs. unmodified proppant due to thehydrogel layer encapsulating the sand grains. Generally, increasing theshear rate or time can cause the bed height of self-suspending proppantto decrease as a result of desorption of the hydrogel layer from thesurface of the modified proppant. For this reason, it is desirable forthe bed height to be as high as possible in this test, especially aftershear. The results below show that the addition of glycerol improves thebed height and the shear stability of the product. The addition ofglycerol and Ethal, while it improves the initial bed height, the longterm shear stability is slightly decreased. These results areillustrated in the graph in FIG. 6.

Example 13 Glycerol and Processability

This experiment sought to determine the effect of glycerol and otheradditives on the performance of self-suspending proppants (denoted asSSP below). 1 kg of dry 40/70 sand was added to the bowl of a KitchenAidstand mixer, model KSM90WH, which was fitted with the paddle attachment.3.09 g of glycerol was mixed with 27.84 g of EM533 emulsion polymer,then the mixture was added to the top of the sand and allowed to soak infor 1 minute. At time 0, the mixer was started at speed 1 (72 rpmprimary rotation). Samples were collected at 1-2 minute intervals anddried for 1 hour at 90° C. Then, each sample was subjected to the 1liter shear test, where 50 g of SSP was added to 1 L of water andsheared at 200 rpm (an approximate shear rate of 550 s⁻¹) for 20minutes. After transferring the water/SSP mixture to a 1 liter graduatedcylinder and settling for 10 min, the bed heights were recorded. Theexperiment was repeated with 30.93 g EM533 emulsion polymer alone addedto 1 kg of sand. These results are shown in FIG. 7. As shown in thegraph, the glycerol additive increased bed heights significantly.

The difference in performance was even more marked when the experimentwas repeated at higher mixing speeds. Here the mixer was set to speed 4(150 rpm primary rotation). At low mixing times, the samples wereinsufficiently mixed, leading to incomplete coating of the sand andready desorption of the polymer from the surface of the SSP during theshear test. As mixing time of the coating process increased so didperformance, until an ideal coating was reached, giving maximum bedheight for that sample. After that, increasingly worse (lower) bedheights were seen at higher mixing times, possibly as a result ofabrasion of the coating during extended mixing. At higher mixing speeds,this process happened even faster, such that the processing window forthe emulsion polymer alone was less than 1 minute. With the addition ofglycerol and the use of a lower mixing speed, this processing window waswidened to nearly 15 minutes. In comparison to the tests with emulsionpolymer alone, glycerol caused the processing window to widen,indicating that SSP preparation with the glycerol is more robust. At thesame time, glycerol allowed the polymer emulsion to invert more fully,leading to better coatings and increased bed heights. Testing withcombinations of glycerol and the emulsion polymer EM533 at a highermixing speed yielded the results shown in FIG. 8.

Example 14 Modified Proppant with an Anticaking Agent

Modified proppant samples were made with and without anticaking agentfor a comparison. For Sample A, 50 g of 40/70 sand was added to aFlackTek jar. 1.5 g of EM533 emulsion polymer was added to the sand andthe sample was mixed for 30 seconds. After mixing, 0.25 g of calciumsilicate was added to the sample and the sample was mixed again for 30seconds. The sample was then dried for 1 hour at 85° C. After drying,the sample was poured over a 25 mesh screen and shaken lightly for 30seconds. The amount of sand that passed through the sieve was thenmeasured. For Sample B, 50 g of 40/70 sand was added to a FlackTek jar.1.5 g of EM533 emulsion polymer was added to the sand and the sample wasmixed for 30 seconds. The sample was then dried for 1 hour at 85° C.After drying, the sample was poured over a 25 mesh screen and shakenlightly for 30 seconds. The amount of sand that passed through the sievewas then measured. Table 8 shows the results.

TABLE 8 Total Mass Mass passing % Passing Sample Sample, g Sieve, gSieve Sample A 50.5 50.16 99.3% Sample B 50.5 15.71 31.1%

The results of sieve testing show that the incorporation of ananticaking agent was effective at improving the handling properties ofthe modified proppants.

Samples A and B were separately added to 1 L of water and then shearedin the EC Engineering Mixer for 20 minutes at 200 rpm. After shearing,the samples were transferred to a 1 L graduated cylinder and left tosettle for 10 minutes. After settling, the bed heights were measured,showing no significant loss in shear stability as a result ofincorporating an anticaking agent. Table 9 shows these results.

TABLE 9 Sample Bed Height, mm Sample A 56.21 Sample B 59.67

Example 15 Hydrogel Coating of Sand by Dissolving a Water-SolublePolymer in a Monomer Formulation Followed by Polymerization of theMonomers

2.5 g of a mixture of acrylic acid (Aldrich 147230), poly(ethyleneglycol) methyl ether acrylate (Aldrich 454990), and polyethylene glycoldimethacrylate (Aldrich 437441) in a mol ratio: 0.5/0.4/0.1 can be mixedwith 7.5 g of polyethylene glycol (Aldrich 202371) and 1 wt % ofammonium persulfate. The solution can be mixed with 100 g of 30/70 meshsand under nitrogen and can be allowed to react by increasing thetemperature to 70° C. for 5 hours. Next the solids obtained are washedwith methanol, vacuum filtered and dried in an oven at 80° C.

Example 16 Polyurethane Hydrogel Coating of Sand

100 g of 30/70 mesh frac sand can be added to a Hobart type mixer andheated to 120° C. Next 6 g of a polyethyleneglycol (Fluka 81190) will beadded and allowed to mix for 1 minute. Then 0.53 g of Desmodur N75 fromBayer will be added. After mixing for 1 more minute, one drop ofcatalyst 1,4-Diazabicyclo[2.2.2]octane (Aldrich D27802) will be addedand the mixture will be allowed to react for 5 more minutes. Theobtained solid is washed with methanol, vacuum filtered and dried in anoven at 80° C.

Example 17 Shear Stability Testing

Coated sand samples made in Examples 15 and 16 were tested for shearstability. 1 L of tap water was added to a square beaker with a capacityof 1.25 L and markings at the 1 L level. The beaker was then placed inan EC Engineering CLM4 paddle mixer. The mixer was set to mix at 300rpm. Once mixing commenced, 50 g of the coated sand sample was added tothe beaker. After 30 seconds of mixing at 300 rpm, the mixing wasreduced to 200 rpm and continued for 20 minutes. At the end of thismixing, the mixture was poured into a 1 L graduated cylinder and allowedto settle. After 10 minutes, the settled bed height was recorded, asshown in Table 10. Higher bed heights indicate better proppantperformance.

TABLE 10 Sand Sample Bed Height after shear, mm Untreated 40/70 Sand13.24 Example 2 70.4 Example 3 57.64

Example 18 Brine Tolerance

Two 20 mL vials were filled with 10 mL of tap water. Separately, anothertwo 20 mL vials were filled with 10 mL of a 1% KCl solution. 1 g of sandprepared in Example 15 was added to a vial containing tap water and 1 gwas added to a vial containing 1% KCl. Also, 1 g of sand prepared inExample 6 was added to a vial containing tap water and 1 g was added toa vial containing 1% KCl. All four vials were inverted ˜7 times and thenleft to settle for 10 minutes. After settling, the bed heights weremeasured. The results are shown in Table 11.

TABLE 11 Sand Sample Tap Water Bed Height, mm 1% KCl Bed Height, mmExample 2 10.39 5.02 Example 6 17.15 9.23

Example 19 Abrasion Testing

Three 250 mL beakers were filled with 50 mL of tap water. One aluminumdisk with a mass of about 5.5-6 g was placed in each of the beakers. One2 inch stir bar was placed in each of the beakers as well. All threebeakers were placed their own magnetic stir plates and the plates wereset to speed setting 5. Six grams of 40/70 sand was added to one of thebeakers. Six grams of sand prepared in Example 15 was placed a secondbeaker. The third beaker had no sand added at all. Each of the beakerswas left to stir for 2 hours. After stirring, the aluminum disk wasremoved, washed and then dried. The mass was then measured again. Theresults, shown in Table 12, indicate that the sand prepared in Example15 results in less abrasion to metal surfaces upon contact, comparedwith unmodified sand.

TABLE 12 Initial Total Mass, g Mass After 2 hrs, g Loss, g % Loss NoSand 5.62 5.612 0.008 0.14% 40/70 Sand, uncoated 6.044 6.027 0.017 0.28%Example 15 Sand 5.673 5.671 0.002 0.04%

Example 20 Effect of Glycerol on Mixing

1 kg of dry 40/70 sand was added to the bowl of a KitchenAid standmixer, model KSM90WH, which was fitted with the paddle attachment. 3.09g of glycerol was mixed with 27.84 g of emulsion polymer then themixture was added to the top of the sand and allowed to soak in for 1minute. At time 0 the mixer was started at speed 4 (150 rpm primaryrotation). Samples were collected at 1-2 minute intervals and dried for1 hour at 90° C. Then, each sample was subjected to a shear test, where50 g of SSP was added to 1 L of water and sheared at 550 s⁻¹ for 20minutes. After settling for 10 min, the bed heights were recorded. Theresults of these shear tests are shown in FIG. 9. The graph demonstratesthat both undermixing and overmixing can affect the behavior of thecoated proppants, leading to dissociation of the polymer from the sandduring the shear test. In this example, an optimal amount of mixing wasbetween about 5 and 20 minutes. The effect of mixing duration uponperformance suggests that the coating is fragile while wet, and it ismore durable once it is dry. In comparison to the coating tests withemulsion polymer alone, coatings with glycerol-blended emulsionsappeared to cause the processing window (i.e., the acceptable amount ofmixing time) to widen. Additionally, glycerol-blended emulsion coatingsappeared to invert more fully, leading to better coating properties suchas increased bed heights.

Example 21 Production of Self-Suspending Proppant Using a Pug Mill

A 3 cubic foot pug mill type mixer was used to make a batch ofself-suspending proppant. About 50 lbs of 40/70 mesh sand was added tothe pug mill. In a 1 L beaker, about 756 g of SNF Flopam EM533 was addedand 84 g of glycerol was mixed into the polymer. The entire mixture wasthen poured evenly on top of the sand in the pug mill. The pug mill wasturned on and mixed at about 70 rpm. Samples were taken after 30, 60,120, 180, 240, 300, 420, and 600 seconds of mixing. The samples weredried for one hour. After drying, the 50 g of each sample was added to 1L of water and mixed in an EC Engineering CLM4 for 20 min at 200 rpm.After mixing, the sample was poured into a 1 L graduated cylinder andallowed to settle for 10 minutes. After settling, the bed height wasmeasured. The results are shown in Table 13.

TABLE 13 Pug Mill Mixing Time (sec) Bed Height, mm 30 29.34 60 23.49 12048.9 180 57.58 240 55.71 300 44.88 420 57.21 600 57.25

Example 22 Wet Aging

A 400 g sample of self-suspending proppant (SSP) was manufactured in thesame manner as Example 15. The 400 g of SSP was split into 50 g samplesand left in closed containers and left at room temperature. After dryingfor various amounts of time, the samples were tested in the same manneras Example 21. The results are shown in Table 14.

TABLE 14 Aging Time, hr Final Bed Height, mm 0 10.1 2 26.63 4 60.16

Example 23 SSP Plus Uncoated Proppant

10 mL of tap water was added to a 20 mL vial. Proppant sand, both SSPprepared in accordance with Example 15 and unmodified 40/70 was thenadded to the vial. The vial was inverted several times and then left tosettle for 10 minutes. After settling, the bed height was measured. Theresults are shown in Table 15.

TABLE 15 SSP, grams 40/70 Sand, grams Settled Bed Height, mm 0.5 0.55.46 0.75 0.25 5.71 0.9 0.1 8.23

Example 24 Anti-Caking Agents Added to SSP

A 400 g batch of SSP was produced in the same manner as described inExample 15. The sample was split into about 50 g subsamples and then0.25 g of fumed silica with an aggregate size of 80 nm was mixed intoeach sample. Samples were then covered and aged at room temperature. Thesamples were tested in the same manner as described in Example 21. Theresults are shown in Table 16.

TABLE 16 Hours Aging Settled Bed Height, mm 18 57.3 24 41.28 42 44.29 4844.76 72 45.48

Example 25 Respirable Dust

200 g samples of uncoated and hydrogel-coated sand (40/70 mesh) preparedaccording to Example 15 were sieved with a 140 mesh screen, and the fineparticulates that pass through the 140 mesh sieve were collected andweighed. The coated sample of sand demonstrated an 86% reduction on theamount of fine particulates relative to the uncoated sample of sand. Theresults are shown in Table 17.

TABLE 17 Weight of Weight of Dust Percentage the sample the dust oftotal Uncoated sample 200.011 g 0.0779 g 0.03895% Coated sample 200.005g 0.0108 g 0.00540%

Example 26 Anti-Caking Agents with Different Particulate Size

50 g of 40/70 mesh sand was mixed with 2 g of SNF Flopam EM533 using thespeed mixer for 30 seconds at 800 rpm. Then 0.625 g of an anti-cakingagent was added and the material was again mixed in the speed mixer for30 seconds. The samples were allowed to sit for 3 hours, then tested ina 20 min shear test, allowed to settle for 10 min and the bed heightmeasured. Results are shown in Table 18. The anti-caking agents improvedthe bed height after shear testing with a wide range of particle sizes.

TABLE 18 Anti-caking agent Particulate Size Bed Height (mm) Talc(magnesium 12 microns 16.76 silicate) Calcium Silicate 1-3 microns 39.78Fumed Silica (EH-5) 80 nanometers 73.87

Example 27 Chemical Composition of Anti-Caking Agents

A wide variety of anti-caking agents were tested, as listed in Table 19.For each agent tested, 700 g of 40/70 sand was mixed in the KitchenAidmixer at speed 1 (144 rpm) with 21.65 g of a 10% glycerol/90% EM533mixture. 50 g samples were separated out and mixed with the appropriateamount of anti-caking agent in the speed mixer. Three samples, whichwere mixed with 1% calcium silicate, 1.5% diatomaceous earth, and 1.5%Kaolin respectively, were tested in the shear test immediately, whilethe other 7 samples were dried for 1 hour in an 80° C. oven along with acontrol sample with no anti-caking agent. All samples were tested in thesame manner as Example 17. Table 19-A shows bed heights after sheartesting wet (non-dried) samples with an anti-caking agent applied. Table19-B shows bed height after shear testing of dried (1 hr at 80° C.)samples with anti-caking agent applied.

TABLE 19-A Anti-Caking Agent Amount Bed Height (mm) Calcium Silicate 0.5g (1%)  30.26 Diatomaceous earth (DE) 0.75 g (1.5%) 12.95 Kaolin clay0.75 g (1.5%) 18.46

TABLE 19-B Anti-Caking Agent Amount Bed Height (mm) NONE — 85.9 SodiumBicarbonate 0.5 g (1%) 56.97 Cornstarch 0.5 g (1%) 32.29 Baby Powder(talc) 0.5 g (1%) 84.83 Dry-Floc AF (hydrophobic 0.5 g (1%) 32.24starch) Tapioca Maltodextrin 0.5 g (1%) 27.08 Microcrystalline cellulose0.5 g (1%) 40.12 Baking Powder 0.5 g (1%) 39.88

Example 28 Anti-Caking Agents: Amounts Needed for Drying

Seven 50 g samples of 40/70 sand were added to small plastic jars,followed by 2 g each of 10% glycerol/90% emulsion polymer mixture foreach. After speed mixing for 30 seconds, 0.25 g, 0.375 g, 0.5 g, 0.675g, 0.75 g, 1 g and 2.5 g of calcium silicate powder were added to theseven samples and the sand was again mixed for 30 seconds. The sampleswere shear tested without a further drying step, and the settled bedheight was recorded in mm. The results are shown in FIG. 10. A similarexperiment was carried out using silica as an anticaking agent. Thesetests showed that a sand coated with a hydrogel can be treated with ananticaking agent, yielding a product that does not require a separatedrying step to produce an acceptable bed height after shear testing.

Example 29 Silica Anti-Caking Agents

50 g of 40/70 sand was added to a small jar, followed by 2 g of 10%glycerol/90% EM533. The jar was speed mixed at 800 rpm for 30 seconds,then the appropriate amount of fumed silica was added, as described inTable 20, and it was mixed for another 30 sec. The samples underwent a20 min shear test and the bed heights were recorded. No oven drying wasused. Results are shown in Table 20.

TABLE 20 Compound name Chemical character Amount added Bed Height EH-5Amorphous fumed 1% 136.25 mm silica M-5 Untreated fumed silica 1% 123.52mm TS-720 Treated fumed silica, 1%  26.21 mm siloxanes and siliconesPG001 30% anionic colloidal 1% solids  15.30 mm silica, 25.9% solids

A batch of coated sand was mixed in the KitchenAid mixer and separatedinto several 50 g samples. Then 1 wt % of various sizes of fumed silicawere added to each of 3 samples, mixed, and shear tested. These testresults are shown in Table 21.

TABLE 21 Powder Approx. Size Amount added Bed height Aldrich Fumed  7 nm1% 48.86 mm Silica Aldrich Silica 10 nm 1% 35.48 mm Nanopowder CabotEH-5 80 nm aggregates 1% 59.10 mm

Example 30 Preheating Sand

500 g of 30/50 sand was placed in a 90° C. oven for 1 h with occasionalstirring, until the temperature of the sand equilibrated. The sand wasthen mixed in a commercial planetary mixer until it reached the desiredpre-heated temperature (45° C., 60° C. or 80° C.), at which point 20.8 gof the SNF Flopam EM533 was added and the sample mixed for 7 min. Thebatch was then divided and dried in the oven for a range of times at 80°C. For the non-preheated samples, 500 g of 30/50 sand was placed in themixer bowl with 20.8 g of polymer emulsion, mixed for 7 minutes, andthen dried for varying amounts of time. All samples were shear testedusing the standard procedure: 50 g of sand added to 1000 g of tap water,stirred at a shear rate of 550 s⁻¹ for 20 minutes, then settled for 10min in a 1 L graduated cylinder. The results are shown in FIG. 11. Theseresults suggest that preheating the sand to 45° C. is acceptable but60-80° C. results in lower bed height in shear tests.

Example 31 Forced Air Drying

50 g of 40/70 sand was mixed with of 4% emulsion polymer (2 g) preparedaccording to Example 14 using the speed mixer for 30 seconds. The samplewas transferred to a container fitted with a hot air gun set at 90, 95or 100° C. The sample was left under the heat gun for 30 min total, with5 g samples taken out at the 5, 10, 15 and 30 min marks. These sampleswere then tested using the Small shear test, performed as follows: 100mL of tap water was set stirring in a 300 mL beaker using a 2 inch stirbar spinning at 500 rpm; 5 g of the sand sample was added to the beakerand sheared for 3 minutes; the whole solution was transferred to a 100mL graduated cylinder, inverted once, settled for 5 minutes, and the bedheight measured. The results of these tests are shown in FIG. 12. Asshown in the graph, higher temperatures of the incoming forced aircaused more complete drying and better bed height. To test thesusceptibility of SSP to shear while drying with forced air, a sevenprong rake was pulled back and forth through the sample to simulatelight shear while drying. Two 50 g batches of SSP were prepared anddried under 110° C. air for 30 min. The first was completely static,while the second was constantly raked during the 30 min dry time. Bothsamples were tested using the large shear test for 20 min with asettling time of 10 min. The sample with static drying gave a settledbed volume of 100.63 mm; while the sample dried with light shear gave asettled bed volume of 109.49 mm.

Example 32 Mixing with Vertical Screw

A small-scale vertical screw blender was constructed. Sand and SNFFlopam EM533 were added to the container, and then mixed with the screwturning at about 120 rpm. The sample was then split into two 50 g parts,one of which was oven dried at 80° C., the other mixed with 0.5 g (1 wt%) fumed silica. Both were then subjected to a shear test as describedin Example 17. The results of bed height measurement were as follows:Oven Dried, 1 h gave a bed height of 101.34 mm; Undried, with 1% of 7 nmfumed silica added, gave a bed height of 91.47 mm. Both oven drying andthe addition of anti-caking agent to dry the product produced high bedheights.

Example 33 Microwave Drying

50 g of 40/70 sand was added to a small plastic jar, and then mixed with2 g of a blend containing (10% glycerol/90% emulsion polymer) in thespeed mixer for 30 seconds at 800 rpm. The sample was placed in a 700 Wmicrowave oven and heated on high for 45 seconds. The sample was sievedand cooled, then sheared at 200 rpm for 20 min in an EC Engineering CLM4mixer. After mixing the sample was transferred to a 1 L beaker and leftto settle for 10 minutes. After settling, the bed height was measured inmillimeters, giving a bed height of 52.43 mm. Microwave heating givesacceptable bed heights with relatively short drying times.

Example 34 Mixing and Heating with Anti-Caking Agents

500 g 40/70 sand was mixed in a KitchenAid mixer with 20 g of (20%Glycerol/80% emulsion) for 8 min. Next was added 0.44% of Cabot EH-5fumed silica and mixed for 2 minutes, and then the sample was heatedwith the heat gun. 50 g samples were collected at 13, 18, 24 and 26minutes of mixing time. These were shear tested for 20 min and the bedheights recorded. The results are shown in FIG. 13. A combination ofglycerol and silica made the processing window longer.

Example 35 Microwave Drying

400 g of 30/50 sand was combined with 16 g (4% wt) of emulsion polymerprepared according to Example 14 and mixed in a KitchenAid stand mixerfor 7 minutes. One 50 g sample was dried using the oven (80° C.), and 7other samples were placed in a 700 W microwave oven for 5, 10, 20, 30,45, 60 and 120 seconds respectively. Shear tests (20 minutes long) asdescribed in Example 12 and loss on ignition (LOI) tests were run on thesamples. An LOI test consisted of adding 10 g of sand to a taredcrucible, which was placed in a 960° C. oven for 1 hour. After heatingfor an hour, the crucible was cooled in a dessicator for 1 hour thenweighed. Drying time, bed height and LOI are shown on Table 22. Thedifference between the initial and final weights was expressed as apercentage of the total initial sand weight, as shown in FIG. 14.

TABLE 22 Drying Method Drying time Bed height (mm) LOI (%) Oven 1 h41.36 1.8 Microwave 5 sec 15.54 3.33 Microwave 10 sec 16.14 Microwave 20sec 24.68 Microwave 30 sec 39.99 2.929 Microwave 45 sec 53.31 Microwave60 sec 49.84 Microwave 120 sec 57.81 2.279

These tests suggest that the microwave drying technique removespredominantly the water, rather than the oil, from the coated samples.

Example 36 Vacuum Drying

250 g of 30/50 sand were combined with 10 g emulsion polymer asdescribed in Example 14. The sand mixture was stirred in a KitchenAidstand mixer on lowest speed for 7 minutes, then separated into 50 gsamples and dried in a vacuum oven under 24 inches Hg vacuum at 25° C.,50° C. and 85° C. for 1 hour, 1 hour, and 30 minutes respectively. Thesamples were cooled to room temperature, sieved, and shear tested (asdescribed in Example 12) for 20 minutes. The results are shown in Table23.

TABLE 23 Sample # Temperature (° C.) Time Bed Height (mm) 1 25 1 hour16.79 2 50 1 hour 17.34 3 85 30 min 18.04

During these tests, none of the samples dried completely, althoughfurther testing may show that higher temperatures can effect morecomplete drying.

Example 37 Hydrogel Coating of Sand by Admicellar Polymerization

250 g of 30/70 frac sand can be added to 500 ml of a previously degassedaqueous solution containing 0.6 mM hexadecyltrimethylammonium bromide(CTAB) surfactant (equivalent to ⅔ of the critical micelle concentrationof CTAB), and 6 mM monomer (mixture of acrylic acid/acrylamide in a molratio 30/70). Adsorption of the CTAB and monomer onto the sand particlecan be carried out under gentle stirring for 24 h at 25° C. Then, 0.6 mMAmmonium persulfate can be added to the reactor and the polymerizationwill take place for 3 h at 80° C. Excess polymer and surfactant can berinsed with several volumes of water and the sample will be driedovernight in the vacuum oven at 80° C.

Example 38 Hydrogel Coating of Sand by Inverse Suspension Polymerization

To a flask can be added 60 ml of DI-water, 6.6 g acrylamide, 3 g ofacrylic acid, 2.4 g of N,N′-methylenebisacrylamide, 0.1 g ammoniumpersulfate, 2.0 g sodium chloride and 2 drops ofN,N,N′,N′-tetramethylethylenediamine. To this solution can be added 200g of 30/70 mesh frac sand and the whole mixture will be kept attemperature <10° C. To the mixture can be added 200 ml of cyclohexaneand the whole mixture can be vigorously stirred under nitrogen. Next thetemperature can be increased to 60° C. and the reaction allowed toproceed for 6 hours. The resultant coated particles can be filtered andwashed with hot water, acetone and dried at 45° C. under reducedpressure.

Example 39 Coating Polymer

A mixture for coating the proppant was made by combining 10 g glyceroland 90 g Flopam EM533 in a glass vial and mixing for 30 seconds with avortex mixer. This polymer mixture is used in following examples.

Example 40 Preparation of 40/70 Mesh Self-Suspending Proppant (“SSP”)

A sample of 40/70 mesh size SSP was prepared by adding 500 g of 40/70frac sand into the bowl of a KitchenAid mixer. 20 g of the coatingpolymer of Example 39 was added to the sand. The mixer was turned on ata setting of 1 and the sand and polymer mixture mixed for 7 minutes.After mixing, the sample was dried for 1 hour at 85° C. After 1 hour,the sample was removed from the oven and any lumps were broken intoindividual grains.

Example 41 Preparation of 30/50 Mesh Self-Suspending Proppant (“SSP”)

A sample of 30/50 mesh size SSP was prepared by adding 500 g of 30/50frac sand into the bowl of a KitchenAid mixer. 20 g of the coatingpolymer of Example 39 was added to the sand. The mixer was turned on ata setting of 1 and the sand and polymer mixture mixed for 7 minutes.After mixing, the sample was dried for 1 hour at 85° C. After 1 hour,the sample was removed from the oven and any lumps were broken intoindividual grains.

Example 42 Reduced Fines Content of Self-Suspending Proppant (“SSP”) vs.Sand

A stack of standard mesh sieves was prepared with 40 mesh on top, 70mesh in the middle, and a pan at the bottom. The tare weight of eachclean/dry sieve was measured and recorded. 50 g of the 40/70 mesh SSP ofExample 20 was added to the top of the sieve stack, and the stack wasshaken by hand for five minutes. After shaking, the stack wasdisassembled and each sieve was weighed. The mass retained on each sievewas calculated as a percent of the original sample mass, and the amountof the sample remaining in the pan represents the fines fraction, asdefined by a 70 mesh cutoff. The procedure was repeated, substitutingunmodified 40/70 mesh frac sand for the 40/70 SSP. The results in Table24 show the particle size distribution for 40/70 SSP. Table 25 containsthe particle size analysis for unmodified 40/70 frac sand. The resultsshow that the amount of material passing the 70 mesh screen is reducedin 40/70 SSP (1.2% vs. 4.8%). This shows that SSP can contain a reducedamount of fine particulates than a sand sample.

TABLE 24 Particle size analysis of 40/70 SSP Sample: 49.516 g of 40/70SSP Size Mesh Tare, g Final Mass, g Mass Sand Retained, g Distribution40 118.826 127.685 8.859 17.9% 70 111.136 151.036 39.9 80.6% Pan 81.501 82.072 0.571 1.2% Total 49.33 99.6%

TABLE 25 Particle size analysis of unmodified 40/70 white sand Sample:50.974 g of 40/70 White sand Final Mass, Mass Sand Size Mesh Tare, g gRetained, g Distribution 40 118.806 118.921 0.115 0.2% 70 111.045159.465 48.420 95.0% Pan 81.503  83.935 2.432 4.8% Total 50.967 100.0%

Example 43 Friction Reduction

1 L of tap water was added to a square beaker and the beaker was placedin an EC Engineering CLM-4 Mixer. The mixer was turned on and set to 200rpm mixing speed. 120 g of the 30/50 SSP of Example 21 was added to thetap water. The slurry mixed for 20 minutes, then was transferred to a 1L graduated cylinder and left to settle for 10 minutes. After settling,the supernatant was collected. This procedure was repeated until 2 L ofsupernatant fluid was collected. The friction reduction of the collectedfluid was then determined using a flow loop apparatus. The flow loopconsists of a 0.12 in (ID) by 3 ft stainless steel test pipe and a pumpthat delivers a constant flowrate of 55 gph. These conditions correspondto a Reynolds number of 23,000, confirming that the fluid is inturbulent flow. The percent friction reduction (% FR) is determinedexperimentally by measuring the pressures at the entrance and the exitof the test pipe at a constant flow rate. The following equation tocalculate the percent friction reduction: % FR=100*(1−(ΔP_(i)/ΔP₀)),where here ΔP_(i) is the pressure drop across the test pipe using theSSP supernatant fluid and ΔP₀ is the pressure drop across the test pipeusing tap water. The pressure values were ΔP_(i)=11.8 psi and ΔP₀=38.5psi, corresponding to a friction reduction (% FR) of 69%. This showsthat the SSP contributes significantly to friction reduction of theassociated fluid, representing a reduction in the pumping requirements.

Example 44 Hydraulic Conductivity Tests

To model hydraulic conductivity of a simulated proppant pack, 48 g of30/50 mesh size SSP of Example 21 were mixed into 1 liter of water.Ammonium persulfate, was added at 0.1% level and the mixture was heatedto 185° F. for 1 hour. After cooling to room temperature, the mixturewas filtered through a 2.25 inch ID column with a 100 mesh sieve at thebottom, separating the particles from the fluid. The particles formed abed depth of 0.5 inch on the 100 mesh screen, and the flowrates ofvarious fluids through the bed were measured by gravity flow. A plainsand bed was constructed in a similar manner and the flowrates comparedwith the SSP derived bed. Using this method, the flowrates obtained bySSP (250 mL efflux in 28 seconds) and plain sand (250 mL efflux in 25seconds) were nearly identical, showing that SSP, once treated withoxidative breakers such as ammonium persulfate, has no deleteriouseffect on hydraulic conductivity of a sand bed or a simulated proppantpack.

Example 45 Self-Suspending Proppants (“SSP”) with Anticaking Agents

In addition to anticaking agents' being able to replace a drying step,they can be used to generally improve handling qualities for SSP. Anumber of different particulate materials were tested as anticakingagents, as set forth in Table 26 below. To prepare the samples for eachmaterial, 800 g of 30/50 mesh sand was mixed in a KitchenAid mixer atspeed 1 (144 rpm) with 32 g of coating polymer of Example 19.20 gsamples were taken and blended with a selected anticaking agent in amixer, with anticaking agent doses calculated as a percent of the totalsand in the sample. The consistency of the samples was observed andrecorded as “Appearance before drying,” then they were dried for 1 hourat 85° C. Their consistency was again observed and recorded as“Appearance after drying.” Samples were then subjected to conditions of80%-90% relative humidity at 25-35° C. for one hour to assess theiranticaking properties, and consistency was observed and recorded as“Appearance after humidity exposure.” Results are shown in Table 26below, indicating that the anticaking agent improves the handlingproperties of the SSP, where free-flowing is a desirable feature.

TABLE 26 Evaluation of SSP samples with added anticaking agentsAppearance Appearance Anticaking Anticaking before Appearance afterhumidity agent dose agent drying after drying exposure 0 None Wet ClumpySticky (control) 0.1% Fumed Silica Wet Slightly Free-flowing clumpy 0.2%Fumed Silica Slightly Free-flowing Free-flowing clumpy 0.5% Fumed SilicaFree-flowing Free-flowing Free-flowing 0.5% Calcium Free-flowingFree-flowing Free-flowing Silicate   1% Corn Starch Wet Clumpy Clumpy  1% Sodium Wet Clumpy Slightly Stearate clumpy 0.5% Kaolin SlightlyFree-flowing Free-flowing clumpy 0.5% Bentonite Slightly Free-flowingFree-flowing clumpy 0.2% Attapulgite Slightly Free-flowing Free-flowingclumpy

Example 46 Treating Resin Coated Sand with SMA 4000i

Resin coated sand was coated with SMA 4000i by adding 25 g of resincoated sand into a 250 mL round bottom flask. Separately, 0.25 g of SMA4000i was dissolved in 3.57 g of tetrahydrofuran (THF) to make a 7%solution. 1.43 g of the THF solution was then added to the resin coatedsand in the round bottom flask. Additional THF was added to the roundbottom flask until the sand was coved. The THF was then evaporated offof the sample using a rotary evaporator.

Example 47 Treating Resin Coated Sand with SMA 4000i

Resin coated sand was coated with SMA 4000i by adding 25 g of resincoated sand into a 250 mL round bottom flask. Separately, 0.25 g of SMA2000i was dissolved in 3.57 g of THF to make a 7% solution. 0.72 g ofthe THF solution was then added to the resin coated sand in the roundbottom flask. Additional THF was added to the round bottom flask untilthe sand was coved. The THF was then evaporated off of the sample usinga rotary evaporator.

Example 48 Treating Resin Coated Sand with SMA 2000i

Resin coated sand was coated with SMA 2000i by adding 25 g of resincoated sand into a 250 mL round bottom flask. Separately, 0.25 g of SMA4000i was dissolved in 3.57 g of THF to make a 7% solution. 1.43 g ofthe THF solution was then added to the resin coated sand in the roundbottom flask. Additional THF was added to the round bottom flask untilthe sand was coved. The THF was then evaporated off of the sample usinga rotary evaporator.

Example 49 Treating Resin Coated Sand with SMA 2000i

Resin coated sand was coated with SMA 2000i by adding 25 g of resincoated sand into a 250 mL round bottom flask. Separately, 0.25 g of SMA2000i was dissolved in 3.57 g of THF to make a 7% solution. 0.72 g ofthe THF solution was then added to the resin coated sand in the roundbottom flask. Additional THF was added to the round bottom flask untilthe sand was coved. The THF was then evaporated off of the sample usinga rotary evaporator.

Example 50 Treating Resin Coated Sand with Pluronic L31

Resin coated sand was coated with Pluronic Surfactant L31 by adding 20 gof resin coated sand into a small FlackTek jar. 0.025 g of thesurfactant was added to the resin coated sand. The sample was then mixedusing a FlackTek Speedmixer at 800 rpm for 30 seconds.

Example 51 Treating Resin Coated Sand with Pluronic L35

Resin coated sand was coated with Pluronic Surfactant L35 by adding 20 gof resin coated sand into a small FlackTek jar. 0.025 g of thesurfactant was added to the resin coated sand. The sample was then mixedusing a FlackTek Speedmixer at 800 rpm for 30 seconds.

Example 52 Treating Resin Coated Sand with Pluronic L81

Resin coated sand was coated with Pluronic Surfactant L35 by adding 20 gof resin coated sand into a small FlackTek jar. 0.025 g of thesurfactant was added to the resin coated sand. The sample was then mixedusing a FlackTek Speedmixer at 800 rpm for 30 seconds.

Example 53 Treating Resin Coated Sand with ARQUAD® 2HT-75

Resin coated sand was coated with ARQUAD® 2HT-75 by adding 25 g of resincoated sand into a 250 mL round bottom flask. Separately, 0.25 g ofARQUAD® 2HT-75 was dissolved in 3.57 g of IPA to make a 7% solution.0.72 g of the IPA solution was then added to the resin coated sand inthe round bottom flask. Additional IPA was added to the round bottomflask until the sand was coved. The IPA was then evaporated off of thesample using a rotary evaporator.

Example 54 Treating Resin Coated Sand with ADOGEN® 464

Resin coated sand was coated with ADOGEN® 464 by adding 20 g of resincoated sand into a small FlackTek jar. 0.025 g of the ADOGEN® 464 wasadded to the resin coated sand. The sample was then mixed using aFlackTek Speedmixer at 800 rpm for 30 seconds.

Example 55 Coating Polymer Mixture

9 g of Flopam EM 533 (SNF) was combined with 1 g of glycerol in a 20 mLvial. The vial was then mixed for 30 seconds on a vortex mixer.

Example 56 Hydrogel Coating of Sand Samples

Sand samples were prepared by placing 20 g of the samples prepared inExample 46 through Example 54 into small FlackTek jars. 0.6 g of thecoating mixture prepared in Example 35 was added into each the jar. Thecontents were then mixed at 800 rpm for 1 minute using a FlackTek speedmixer. The samples were then dried for 30 minutes at 100° C. Afterdrying, 1 g of each sample was added to a 20 mL vial containing 10 mL oftap water. The vials were mixed gently for 1 minute then left to settlefor 10 minutes. After settling, the bed height was measured to determinepolymer hydration. The results of the testing are shown in Table 27.

TABLE 27 Settled bed heights Amount on Resin Bed Example Number AdditiveCoated Sand Height 46 SMA 4000i 4000 ppm 10 mm  47 SMA 4000i 2000 ppm 8mm 48 SMA 2000i 4000 ppm 8 mm 49 SMA 2000i 2000 ppm 4 mm 50 Pluronic L311250 ppm 8 mm 51 Pluronic L35 1250 ppm 7 mm 52 Pluronic L81 1250 ppm 10mm  53 ARQUAD ® 2HT-75 2000 ppm 3 mm 54 ADOGEN ® 464 1250 ppm 2 mm

Example 57 Humidity Aging Test (Metal Chelate Crosslinkers)

Tyzor TE is a triethanolamine titanium chelate 80% solution in ethanol.Tyzor TEAZ is a 100% actives triethanolamine zirconium chelate product.These metal chelates were dispersed in castor oil at differentconcentrations and applied to proppant in a second addition step duringthe coating process. Samples of coated proppant were prepared by adding3 g of a blend of Flopam EM533 and glycerol to 100 g of 30/50 meshproppant white sand in a FlackTek Max 100 jar. The samples were thenmixed in a FlackTek Speedmixer at 850 rpm for 30 seconds. Samples werethen removed from the Speedmixer and in some cases treated with a metalchelate/castor oil blend. Samples were then returned to the Speedmixerand mixed at 850 rpm for 30 seconds. Samples were then removed from theSpeedmixer, transferred to a watch glass, and dried at 100° C. for 30minutes in a forced air laboratory oven. After drying, samples weresieved through an 18 mesh screen. For humidity aging 50 g of theformulated samples were placed in Max 100 FlackTek jars and left sittingin a humidity chamber for 1 hour. The relative humidity of the chamberwas kept between 60-70%. After humidification, samples were tested in aCarver Press cell (2.25″ I.D.) with an applied load of 1,000 lbs for 30seconds. Caking of the samples was visually assessed and compared to thecontrol (no second addition). The extent to which samples caked wasscored from 1 to 4 with a score of “1” indicating a solid cake and ascore of “4” indicating a free-flowing, non-caking material. Results areshown in Table 28.

TABLE 28 Caking Results with Metal Chelate Add Sample Metal chelateMetal Chelate Conc. (ppm) Caking Score 1 None 0 1 2 Tyzor TE 600 3 3Tyzor TE 1500 3 4 Tyzor TEAZ 600 2 5 Tyzor TEAZ 900 3

(Caking Scores for Table 14: “1”—Solid cake that can be handled withoutfalling apart, “2”—Mostly solid cake that begins to break as handled,“3”—Cake is crumbly out of press cell, “4”—No cake formation).

Example 58 Humidity Aging Test (Powder Additives)

Samples of coated proppant sand were formulated by adding 3 g of aFlopam EM533/glycerol blend to 100 g of 30/50 mesh proppant white sand.The samples were mixed at 850 rpm for 30 seconds in a FlackTekSpeedmixer. Samples were then removed from the Speedmixer and in somecases treated with a dry powder. Samples were then returned to theSpeedmixer and mixed at 850 rpm for 30 seconds to uniformly distributethe powder through the sample. Samples were then removed from theSpeedmixer, transferred to a watch glass, and dried at 100° C. for 30minutes in a forced air laboratory oven. After drying, samples weresieved through an 18 mesh screen. For humidity aging about 50 g of theformulated samples were placed in Max 100 FlackTek jars and left sittingin a humidity chamber for 1 hour. The relative humidity of the chamberwas kept between 60-70%. After humidification, samples were tested in aCarver Press cell (2.25″ I.D.) with an applied load of 1,000 lbs for 30seconds. Caking of the samples was visually assessed and compared to thecontrol (no second addition). The extent to which samples caked wasscored from 1 to 4 with a score of “1” indicating a solid cake and ascore of “4” indicating a free-flowing, non-caking material, as shown inTable 29.

TABLE 29 Caking Results of Coated Proppant with Powder Additive PowderMelting Powder Conc. Caking Sample Powder Point (° C.) (wt %) Score 6None N/A 0.0% 1 7 Thixcin-R 85 0.5% 2 8 Stearic Acid 70 0.6% 3

(Caking Scores for Table 15: “1”—Solid cake that can be handled withoutfalling apart, “2”—Mostly solid cake that begins to break as handled,“3”—Cake is crumbly out of press cell, “4”—No cake formation).

Example 59 Oil-Based Additives

Several oil-based or relatively hydrophobic materials were tested todetermine their efficacy in decreasing caking in humidified samples ofself-suspending proppant (SSP). Samples were prepared by mixing 300 g of30/50 sand, preheated to 45° C., with 9 g of a 10% glycerol/90% Flopam533 mixture in a KitchenAid mixer at speed 1. After 1 minute of mixing,the second additive (usually at 0.2% by wt sand) was introduced and themixture was mixed for another minute. The sample was dried under mediumshear conditions using a heat gun and KitchenAid. The samples were thensubjected to >50% RH in a humidity chamber for 1 hour. They were thenindividually tested for caking behavior by undergoing the compressiontest. This consisted of being compressed at 200 PSI for 30 seconds in acompression cell using a Carver Press, then removed from the cell andobserved. The resulting cake (See Table 16, Compression Test) was gradedon the following scale: “1”—Solid cake that can be handled withoutfalling apart, “2”—Mostly solid cake that begins to break as handled,“3”—Cake is crumbly out of press cell, “4”—No cake formation, as setforth in Table 30.

TABLE 30 Anti-caking Results of Coated Proppant with Oil Based Additives2^(nd) Addition: 2^(nd) Addition: Amount (% of Compression ChemicalIdentity total sand weight) Test Control — 1 Castor Oil 0.2% 1 Triacetin(mixed into polymer) 0.3% 1 Grapeseed Oil 0.2% 1 50% Corn Oil mixed with50% D400 0.2% 1 Jeffamine Adogen 464 0.2% 3 50% Adogen 464 mixed with50% 0.2% 2 Castor Oil 10% Adogen 464 mixed with 90% 0.2% 1 Castor OilPetroleum Jelly 0.2% 1 Mineral Oil (high molecular weight) 0.2% 1 CornOil 0.2% 1 Dimethyl, phenylmethyl siloxane, 0.2% 1 trimethyl terminatedAminopropyl terminated poly(dimethyl 0.2% 3 siloxane) 50% Adogen mixedwith 50% Corn Oil 0.2% 2 Arquad 2HT-75 0.2% 3 Adogen 464 0.1% 3

The sample treated with Adogen 464 barely formed a cake in this test,even at lower doses.

EQUIVALENTS

While specific embodiments of the subject invention have been disclosedherein, the above specification is illustrative and not restrictive.While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. Many variations of the inventionwill become apparent to those of skilled art upon review of thisspecification. Unless otherwise indicated, all numbers expressingreaction conditions, quantities of ingredients, and so forth, as used inthis specification and the claims are to be understood as being modifiedin all instances by the term “about.” Accordingly, unless indicated tothe contrary, the numerical parameters set forth herein areapproximations that can vary depending upon the desired propertiessought to be obtained by the present invention.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

The invention claimed is:
 1. A modified proppant, comprising a proppantparticle substrate and a coating of a hydrogel-forming polymer, whereinthe coating localizes on the surface of the proppant particle substrateto produce the modified proppant; wherein the proppant particlesubstrate comprises sand; wherein the coating comprises ahydrogel-forming polymer having a weight average molecular weight of≧about 1 million g/mol, wherein the hydrogel-forming polymer comprisespolyacrylamide; and wherein the modified proppant is made by an invertemulsion coating technique in which the proppant particle substrate iscombined with an invert emulsion in which the oil phase forms thecontinuous phase of the emulsion and a solution or dispersion of thehydrogel-forming polymer in water forms the discontinuous, emulsifiedphase; wherein the hydrogel-forming polymer forms a substantiallycontinuous film on the surface of the proppant particle which iscrosslinked.
 2. The modified proppant of claim 1, wherein the shearingratio of the proppant as determined by a Shear Analytical Test is ≧0.6.3. The modified proppant of claim 1, wherein the modified proppantundergoes a volumetric expansion of at least 100% upon hydration in anexcess of water.
 4. The modified proppant of claim 1, wherein themodified proppant undergoes a volumetric expansion of at least 500% uponhydration in an excess of water.
 5. The modified proppant of claim 1,wherein the amount of coating is less than about 5 wt % of the total dryweight.
 6. The modified proppant of claim 1, further comprising analcohol selected from the group consisting of ethylene glycol, propyleneglycol, glycerol, propanol, and ethanol.
 7. The modified proppant ofclaim 1, wherein the weight average molecular weight of thehydrogel-forming polymer is ≧about 5 million g/mol.
 8. The modifiedproppant of claim 7, wherein a crosslinking agent is applied to thesubstantially continuous film on the surface of the proppant particle.9. The modified proppant of claim 8, wherein the crosslinking agent iscovalent.
 10. The modified proppant of claim 1, wherein the proppant isfree-flowing when dry or after being subjected to a relative humidity ofbetween about 80%-90% for one hour at 25-35° C.
 11. The modifiedproppant of claim 10, wherein a crosslinking agent is applied to thesubstantially continuous film on the surface of the proppant particle.12. The modified proppant of claim 11, wherein the crosslinking agent isa covalent crosslinking agent.
 13. The modified proppant of claim 1,wherein the proppant is dry.
 14. The modified proppant of claim 13,wherein a crosslinking agent is applied to the substantially continuousfilm on the surface of the proppant particle.
 15. The modified proppantof claim 14, wherein the crosslinking agent is a covalent crosslinkingagent.
 16. The modified proppant of claim 1, wherein a crosslinkingagent is applied to the substantially continuous film on the surface ofthe proppant particle.
 17. The modified proppant of claim 16, whereinthe crosslinking agent is a covalent crosslinking agent.
 18. Themodified proppant of claim 17, wherein the substantially continuous filmis crosslinked in an amount sufficient to prevent premature hydration ofthe hydrogel-forming polymer.
 19. The modified proppant of claim 16,wherein the modified proppant is free flowing in 80-90% relativehumidity at 25-50° C.
 20. A hydraulic fracturing formulation, comprisingthe modified proppant of claim 1 and an aqueous carrier liquid.
 21. Amethod of fracturing a well, comprising: introducing the hydraulicfracturing formulation of claim 20 into the well in an effective volumeand at an effective pressure for hydraulic fracturing, therebyfracturing the well.
 22. In a process for fracturing a geologicalformation penetrated by a well in which a fracing fluid containing aproppant is charged into the geological formation with pulsed pressure,a method for reducing the amount of thickening agent that is added tothe fracing fluid comprising selecting as the proppant the modifiedproppant of claim
 1. 23. The method of claim 22, wherein the modifiedproppant hydrates essentially completely within 2 hours of first beingcombined with the fracing fluid.
 24. The method of claim 23, wherein themodified proppant hydrates essentially completely within 10 minutes offirst being combined with the fracing fluid.